Fair Value of Financial Assets and Liabilities - Unrealized Losses on Investments by Investment Category and Length of Time in Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	$ 5,676	$ 4,375
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(40)	(35)
12 Months or Greater, Fair Value	146	30
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(19)	(4)
Total, Fair Value	5,822	4,405
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(59)	(39)
U.S. government and agency obligations
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	1,921	1,168
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(8)	(2)
12 Months or Greater, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Total, Fair Value	1,921	1,168
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(8)	(2)
U.S. corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	1,818	1,383
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(10)	(4)
12 Months or Greater, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Total, Fair Value	1,818	1,383
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(10)	(4)
High yield corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	181	244
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(14)	(16)
12 Months or Greater, Fair Value	33	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(8)	0
Total, Fair Value	214	244
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(22)	(16)
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0
12 Months or Greater, Fair Value	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Total, Fair Value	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	0
Auction rate securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
12 Months or Greater, Fair Value	25	27
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(2)	(2)
Total, Fair Value	25	27
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(2)	(2)
Foreign debt securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	1,753	1,563
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(8)	(4)
12 Months or Greater, Fair Value	86	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1)	0
Total, Fair Value	1,839	1,563
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(9)	(4)
Publicly traded equity securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	3	17
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	(9)
12 Months or Greater, Fair Value	2	3
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(8)	(2)
Total, Fair Value	5	20
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (8)	$ (11)