Fair Value Disclosures - Investments in Certain Entities Calculate Net Asset Value per Share (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 690	$ 733
Total Unfunded Commitments	309	286
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]
Fair Value	$ 14	$ 19
Hedge Funds/Funds of Hedge Funds [Member] \| Maximum [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
Real Assets Funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Unfunded Commitments	$ 128	$ 117
Real Assets Funds [Member] \| Consolidated Variable Interest Entities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	36	34
Total Unfunded Commitments	48	49
Private Equity Funds Of Funds [Member] \| Consolidated Variable Interest Entities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	56	59
Total Unfunded Commitments	18	20
Equity Method Investments [Member] \| Hedge Funds/Funds of Hedge Funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	168	230
Total Unfunded Commitments	$ 64	$ 48
Redemption Frequency (Daily)	30.00%	21.00%
Redemption Frequency (Monthly)	30.00%	21.00%
Redemption Frequency (Quarterly)	29.00%	49.00%
Redemption Frequency (Not Redeemable)	41.00%	30.00%
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]
Fair Value	$ 99	$ 94
Total Unfunded Commitments	85	86
Equity Method Investments [Member] \| Real Assets Funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	302	282
Total Unfunded Commitments	$ 79	$ 69
Redemption Frequency (Quarterly)	78.00%	83.00%
Redemption Frequency (Not Redeemable)	22.00%	17.00%
Redemption Notice Period, Not Redeemable	60 days	60 days
Equity Method Investments [Member] \| Real Assets Funds [Member] \| Maximum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Redemption Notice Period, days		60 days
Equity Method Investments [Member] \| Other Types Of Investments [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 15	$ 15
Total Unfunded Commitments	$ 15	$ 14
Redemption Frequency (Daily)	79.00%	80.00%
Redemption Frequency (Monthly)	79.00%
Redemption Frequency (Not Redeemable)	21.00%	20.00%
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	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).

+ References

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- Definition

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

+ References

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- Definition

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

+ References

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- Definition

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

+ References

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- Definition

Fair value of investment in certain entities that calculate net asset value per share. Includes, but is not limited to, by unit, membership interest, or other ownership interest. Investment includes, but is not limited to, investment in certain hedge funds, venture capital funds, private equity funds, real estate partnerships or funds. Excludes measurement of net asset value as estimate of fair value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=SL6742756-110258

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- Definition

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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=SL6742756-110258

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- Definition

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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- Definition

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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=SL6742756-110258

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