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, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	$ 4,375	$ 2,559
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	35	11
12 Months or Greater, Fair Value	30	128
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	4	4
Total, Fair Value	4,405	2,687
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	39	15
U.S. government and agency obligations
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	1,168	1,161
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2	3
12 Months or Greater, Fair Value	0	20
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Total, Fair Value	1,168	1,181
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	2	3
U.S. corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	1,383	532
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	4	2
12 Months or Greater, Fair Value	0	17
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Total, Fair Value	1,383	549
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	4	2
High yield corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	244	120
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	16	3
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	244	120
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	16	3
Municipal obligations
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value		51
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		51
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	27	60
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2	3
Total, Fair Value	27	60
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	2	3
Foreign debt securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	1,563	695
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	4	3
12 Months or Greater, Fair Value	0	6
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Total, Fair Value	1,563	701
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	4	3
Publicly traded equity securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	17	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	9	0
12 Months or Greater, Fair Value	3	25
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2	1
Total, Fair Value	20	25
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 11	$ 1