Fair Value of Financial Assets and Liabilities - Composition of Investments (Detail) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
U.S. government and agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,639	$ 2,449
Unrealized Gains	8	0
Unrealized (Losses)	(1)	(8)
Aggregate Fair Value	1,646	2,441
U.S. corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,172	2,257
Unrealized Gains	14	1
Unrealized (Losses)	0	(10)
Aggregate Fair Value	2,186	2,248
High yield corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	68	307
Unrealized Gains	1	2
Unrealized (Losses)	(2)	(22)
Aggregate Fair Value	67	287
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8	20
Unrealized Gains	0	0
Unrealized (Losses)	0	0
Aggregate Fair Value	8	20
Municipal obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	517	731
Unrealized Gains	0	1
Unrealized (Losses)	0	0
Aggregate Fair Value	517	732
Auction rate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		27
Unrealized Gains		0
Unrealized (Losses)		(2)
Aggregate Fair Value		25
Foreign debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,236	2,332
Unrealized Gains	11	0
Unrealized (Losses)	0	(9)
Aggregate Fair Value	2,247	2,323
Fixed income securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,640	8,123
Unrealized Gains	34	4
Unrealized (Losses)	(3)	(51)
Aggregate Fair Value	6,671	8,076
Publicly traded equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	113	126
Unrealized Gains	18	40
Unrealized (Losses)	(8)	(8)
Aggregate Fair Value	123	158
Fixed income and publicly traded equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,753	8,249
Unrealized Gains	52	44
Unrealized (Losses)	(11)	(59)
Aggregate Fair Value	$ 6,794	$ 8,234