Fair Value of Financial Assets and Liabilities - Composition of Investments (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
U.S. government and agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 2,261	$ 1,951
Unrealized Gains	5	2
Unrealized (Losses)	(1)	(2)
Aggregate Fair Value	2,265	1,951
U.S. corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,390	1,998
Unrealized Gains	6	1
Unrealized (Losses)	(1)	(4)
Aggregate Fair Value	2,395	1,995
High yield corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	394	570
Unrealized Gains	8	9
Unrealized (Losses)	(7)	(16)
Aggregate Fair Value	395	563
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	87	53
Unrealized Gains	0	0
Unrealized (Losses)	0	0
Aggregate Fair Value	87	53
Municipal obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	960	948
Unrealized Gains	2	2
Unrealized (Losses)	0	0
Aggregate Fair Value	962	950
Auction rate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	29	29
Unrealized Gains	0	0
Unrealized (Losses)	(2)	(2)
Aggregate Fair Value	27	27
Foreign debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,664	2,566
Unrealized Gains	5	2
Unrealized (Losses)	(1)	(4)
Aggregate Fair Value	2,668	2,564
Fixed income securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,785	8,115
Unrealized Gains	26	16
Unrealized (Losses)	(12)	(28)
Aggregate Fair Value	8,799	8,103
Publicly traded equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	201	117
Unrealized Gains	98	103
Unrealized (Losses)	(14)	(11)
Aggregate Fair Value	285	209
Fixed income and publicly traded equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,986	8,232
Unrealized Gains	124	119
Unrealized (Losses)	(26)	(39)
Aggregate Fair Value	$ 9,084	$ 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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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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