Fair Value and Gross Unrealized Losses on Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Investments, Unrealized Loss Position
Less than 12 months Fair Value	$ 6,298	$ 7,849
Less than 12 months Unrealized Losses	(70)	(156)
12 months or more Fair Value	160	137
12 months or more Unrealized Losses	(2)	(3)
Total Fair Value	6,458	7,986
Total Unrealized Losses	(72)	(159)
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Investments, Unrealized Loss Position
Less than 12 months Fair Value	5,323	7,221
Less than 12 months Unrealized Losses	(64)	(150)
12 months or more Fair Value		1
Total Fair Value	5,323	7,222
Total Unrealized Losses	(64)	(150)
Other Debt Securities
Investments, Unrealized Loss Position
Less than 12 months Fair Value	942	595
Less than 12 months Unrealized Losses	(5)	(5)
12 months or more Fair Value	156	132
12 months or more Unrealized Losses	(2)	(3)
Total Fair Value	1,098	727
Total Unrealized Losses	(7)	(8)
Other Debt And Equity Securities [Member]
Investments, Unrealized Loss Position
Less than 12 months Fair Value	33	33
Less than 12 months Unrealized Losses	(1)	(1)
12 months or more Fair Value	4	4
Total Fair Value	37	37
Total Unrealized Losses	$ (1)	$ (1)