Fair Value of Financial Assets and Liabilities - Composition of Investments (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
U.S. government and agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 2,376	$ 1,951
Unrealized Gains	5	2
Unrealized (Losses)	(1)	(2)
Aggregate Fair Value	2,380	1,951
U.S. corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,549	1,998
Unrealized Gains	4	1
Unrealized (Losses)	(5)	(4)
Aggregate Fair Value	2,548	1,995
High yield corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	370	570
Unrealized Gains	2	9
Unrealized (Losses)	(22)	(16)
Aggregate Fair Value	350	563
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	30	53
Unrealized Gains	0	0
Unrealized (Losses)	0	0
Aggregate Fair Value	30	53
Municipal obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	850	948
Unrealized Gains	2	2
Unrealized (Losses)	0	0
Aggregate Fair Value	852	950
Auction rate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	27	29
Unrealized Gains	0	0
Unrealized (Losses)	(2)	(2)
Aggregate Fair Value	25	27
Foreign debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,559	2,566
Unrealized Gains	3	2
Unrealized (Losses)	(8)	(4)
Aggregate Fair Value	2,554	2,564
Fixed income securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,761	8,115
Unrealized Gains	16	16
Unrealized (Losses)	(38)	(28)
Aggregate Fair Value	8,739	8,103
Publicly traded equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	178	117
Unrealized Gains	52	103
Unrealized (Losses)	(9)	(11)
Aggregate Fair Value	221	209
Fixed income and publicly traded equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,939	8,232
Unrealized Gains	68	119
Unrealized (Losses)	(47)	(39)
Aggregate Fair Value	$ 8,960	$ 8,312

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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-Name Accounting Standards Codification

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