Investments - Summary of Cost and Carrying Value of Equity and Trading Debt Securities (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Total trading investments, Cost		$ 760
Total equity and trading debt securities, Cost	$ 888
Total trading investments, Carrying Value		835
Total equity and trading debt securities, Carrying Value	941
Corporate Debt [Member]
Schedule of Investments [Line Items]
Total trading investments, Cost	166	152
Total trading investments, Carrying Value	169	157
Government Debt [Member]
Schedule of Investments [Line Items]
Total trading investments, Cost	62	72
Total trading investments, Carrying Value	62	73
Asset/Mortgage-Backed Debt [Member]
Schedule of Investments [Line Items]
Total trading investments, Cost	62	56
Total trading investments, Carrying Value	62	56
Deferred Compensation Plan Fund [Member]
Schedule of Investments [Line Items]
Total trading investments, Cost		34
Equity securities at FVTNI, Cost	21
Total trading investments, Carrying Value		56
Equity securities at FVTNI, Carrying Value	34
Equity Securities/Multi-Asset Mutual Funds [Member]
Schedule of Investments [Line Items]
Total trading investments, Cost		446
Equity securities at FVTNI, Cost	577
Total trading investments, Carrying Value		$ 493
Equity securities at FVTNI, Carrying Value	$ 614

Marketable equity securities and trading debt securities.

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Marketable equity securities and trading debt securities cost.

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Marketable securities equity securities cost.

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- Definition

For an unclassified balance sheet, this item represents investments in common and preferred stocks and other forms of securities that provide ownership interests in a corporation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(b))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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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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Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26626-111562

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.4)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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Cost of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

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