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	Mar. 31, 2015
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	$ 2,441
Less than 12 Months Gross Unrealized Losses	(20)
12 Months or Greater Fair Value	36
12 Months or Greater Gross Unrealized Losses	(6)
Total Fair Value	2,477
Total, Gross Unrealized Losses	(26)
U.S. government and agency obligations
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	501
Less than 12 Months Gross Unrealized Losses	(1)
12 Months or Greater Fair Value	0
12 Months or Greater Gross Unrealized Losses	0
Total Fair Value	501
Total, Gross Unrealized Losses	(1)
U.S. corporate debt securities
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	778
Less than 12 Months Gross Unrealized Losses	(1)
12 Months or Greater Fair Value	0
12 Months or Greater Gross Unrealized Losses	0
Total Fair Value	778
Total, Gross Unrealized Losses	(1)
High yield corporate debt securities
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	138
Less than 12 Months Gross Unrealized Losses	(7)
12 Months or Greater Fair Value	0
12 Months or Greater Gross Unrealized Losses	0
Total Fair Value	138
Total, Gross Unrealized Losses	(7)
Auction rate securities
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	0
Less than 12 Months Gross Unrealized Losses	0
12 Months or Greater Fair Value	27
12 Months or Greater Gross Unrealized Losses	(2)
Total Fair Value	27
Total, Gross Unrealized Losses	(2)
Foreign debt securities
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	918
Less than 12 Months Gross Unrealized Losses	(1)
12 Months or Greater Fair Value	0
12 Months or Greater Gross Unrealized Losses	0
Total Fair Value	918
Total, Gross Unrealized Losses	(1)
Publicly traded equity securities
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	106
Less than 12 Months Gross Unrealized Losses	(10)
12 Months or Greater Fair Value	9
12 Months or Greater Gross Unrealized Losses	(4)
Total Fair Value	115
Total, Gross Unrealized Losses	$ (14)