Fair Value of Financial Assets and Liabilities - Composition of Investments (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
U.S. government and agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 2,054	$ 2,449
Unrealized Gains	5	0
Unrealized (Losses)	(1)	(8)
Aggregate Fair Value	2,058	2,441
U.S. corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,364	2,257
Unrealized Gains	11	1
Unrealized (Losses)	(2)	(10)
Aggregate Fair Value	2,373	2,248
High yield corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	220	307
Unrealized Gains	3	2
Unrealized (Losses)	(8)	(22)
Aggregate Fair Value	215	287
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13	20
Unrealized Gains	0	0
Unrealized (Losses)	0	0
Aggregate Fair Value	13	20
Municipal obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	640	731
Unrealized Gains	0	1
Unrealized (Losses)	0	0
Aggregate Fair Value	640	732
Auction rate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	25	27
Unrealized Gains	0	0
Unrealized (Losses)	(2)	(2)
Aggregate Fair Value	23	25
Foreign debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,298	2,332
Unrealized Gains	6	0
Unrealized (Losses)	(2)	(9)
Aggregate Fair Value	2,302	2,323
Fixed income securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,614	8,123
Unrealized Gains	25	4
Unrealized (Losses)	(15)	(51)
Aggregate Fair Value	7,624	8,076
Publicly traded equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	113	126
Unrealized Gains	18	40
Unrealized (Losses)	(9)	(8)
Aggregate Fair Value	122	158
Fixed income and publicly traded equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,727	8,249
Unrealized Gains	43	44
Unrealized (Losses)	(24)	(59)
Aggregate Fair Value	$ 7,746	$ 8,234

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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