Fair Value Disclosures - Investments in Certain Entities Calculate Net Asset Value per Share (Detail) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2019	Dec. 31, 2018
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Unfunded Commitments	$ 608	$ 343
Fair Value Measured at NAV per share [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 756	$ 762
Hedge Funds/Funds of Hedge Funds [Member] \| Consolidated Variable Interest Entities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Redemption Notice Period, days	90 days	90 days
Hedge Funds/Funds of Hedge Funds [Member] \| Consolidated Variable Interest Entities [Member] \| Fair Value Measured at NAV per share [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 4	$ 3
Real Assets Funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Unfunded Commitments	152	130
Real Assets Funds [Member] \| Consolidated Variable Interest Entities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Unfunded Commitments	10	37
Real Assets Funds [Member] \| Consolidated Variable Interest Entities [Member] \| Fair Value Measured at NAV per share [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	58	55
Private Equity Funds Of Funds [Member] \| Consolidated Variable Interest Entities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Unfunded Commitments	11	18
Private Equity Funds Of Funds [Member] \| Consolidated Variable Interest Entities [Member] \| Fair Value Measured at NAV per share [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	33	48
Equity Method Investments [Member] \| Hedge Funds/Funds of Hedge Funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Unfunded Commitments	$ 85	$ 96
Redemption Frequency (Daily)	27.00%	30.00%
Redemption Frequency (Monthly)	27.00%	30.00%
Redemption Frequency (Quarterly)	20.00%	18.00%
Redemption Frequency (Not Redeemable)	53.00%	52.00%
Equity Method Investments [Member] \| Hedge Funds/Funds of Hedge Funds [Member] \| Fair Value Measured at NAV per share [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 188	$ 173
Equity Method Investments [Member] \| Hedge Funds/Funds of Hedge Funds [Member] \| Minimum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Redemption Notice Period, days	1 day	1 day
Equity Method Investments [Member] \| Hedge Funds/Funds of Hedge Funds [Member] \| Maximum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Redemption Notice Period, days	90 days	90 days
Equity Method Investments [Member] \| Private Equity Funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Unfunded Commitments	$ 349	$ 83
Equity Method Investments [Member] \| Private Equity Funds [Member] \| Fair Value Measured at NAV per share [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	123	116
Equity Method Investments [Member] \| Real Assets Funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Unfunded Commitments	$ 142	$ 93
Redemption Frequency (Quarterly)	64.00%	68.00%
Redemption Frequency (Not Redeemable)	36.00%	32.00%
Redemption Notice Period, Not Redeemable	60 days	60 days
Equity Method Investments [Member] \| Real Assets Funds [Member] \| Fair Value Measured at NAV per share [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 334	$ 353
Equity Method Investments [Member] \| Other Types Of Investments [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Unfunded Commitments	$ 11	$ 16
Redemption Frequency (Daily)	73.00%	80.00%
Redemption Frequency (Monthly)	73.00%
Redemption Frequency (Not Redeemable)	27.00%	20.00%
Redemption Notice Period, days		5 days
Equity Method Investments [Member] \| Other Types Of Investments [Member] \| Fair Value Measured at NAV per share [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 16	$ 14
Equity Method Investments [Member] \| Other Types Of Investments [Member] \| Minimum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Redemption Notice Period, days	3 days
Equity Method Investments [Member] \| Other Types Of Investments [Member] \| Maximum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Redemption Notice Period, days	5 days

Fair value measured recurring basis using net asset per share value redemption frequency (daily).

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Fair value measured recurring basis using net asset per share value redemption frequency (monthly).

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Fair value measured recurring basis using net asset per share value redemption frequency not redeemable.

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Fair value measured recurring basis using net asset per share value redemption frequency (quarterly).

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Fair value measured recurring basis using net asset per share value redemption notice period not redeemable.

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Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=SL6742756-110258

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 35
-Paragraph 54B
-URI http://asc.fasb.org/extlink&oid=117332851&loc=SL7495116-110257

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Notice period the entity is required to deliver before it can redeem an investment calculated using net asset value per share, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Includes, but is not limited to, investments calculated by per unit, per membership interest, other equity or ownership interest and alternative investments.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=SL6742756-110258

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The amount of unfunded commitments for which the entity is obligated, to those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=SL6742756-110258

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