Fair Value and Gross Unrealized Losses on Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Investments, Unrealized Loss Position
Less than 12 months Fair Value	$ 1,130	$ 2,239
Less than 12 months Unrealized Losses	(11)	(21)
12 months or more Fair Value	60
12 months or more Unrealized Losses	(1)
Total Fair Value	1,190	2,239
Total Unrealized Losses	(12)	(21)
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Investments, Unrealized Loss Position
Less than 12 months Fair Value	898	1,784
Less than 12 months Unrealized Losses	(4)	(18)
Total Fair Value	898	1,784
Total Unrealized Losses	(4)	(18)
Other Debt Securities
Investments, Unrealized Loss Position
Less than 12 months Fair Value	216	454
Less than 12 months Unrealized Losses	(7)	(3)
12 months or more Fair Value	60
12 months or more Unrealized Losses	(1)
Total Fair Value	276	454
Total Unrealized Losses	(8)	(3)
Equity Securities, Other
Investments, Unrealized Loss Position
Less than 12 months Fair Value	16	1
Total Fair Value	$ 16	$ 1