Schedule of Gross Unrealized Losses and Fair Values of Securities (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Investment [Line Items]
Less than 12 months, Fair Value	$ 2,249	$ 527
Less than 12 months, Gross Unrealized Losses	(85)	(5)
12 months or more, Fair Value	57	66
12 months or more, Gross Unrealized Losses	(3)	(3)
Total, Fair Value	2,306	593
Total, Gross Unrealized Losses	(88)	(8)
Debt Securities | Tax-exempt municipal securities
Investment [Line Items]
Less than 12 months, Fair Value	466	233
Less than 12 months, Gross Unrealized Losses	(17)	(3)
12 months or more, Fair Value	28	27
12 months or more, Gross Unrealized Losses	(1)	(1)
Total, Fair Value	494	260
Total, Gross Unrealized Losses	(18)	(4)
Debt Securities | Residential Mortgage-Backed Securities
Investment [Line Items]
Less than 12 months, Fair Value	2	0
Less than 12 months, Gross Unrealized Losses	0	0
12 months or more, Fair Value	2	4
12 months or more, Gross Unrealized Losses	0	(1)
Total, Fair Value	4	4
Total, Gross Unrealized Losses	0	(1)
Debt Securities | Commercial Mortgage-Backed Securities
Investment [Line Items]
Less than 12 months, Fair Value	199	94
Less than 12 months, Gross Unrealized Losses	(7)	0
12 months or more, Fair Value	0	0
12 months or more, Gross Unrealized Losses	0	0
Total, Fair Value	199	94
Total, Gross Unrealized Losses	(7)	0
Debt Securities | Asset-backed Securities
Investment [Line Items]
Less than 12 months, Fair Value	36	2
Less than 12 months, Gross Unrealized Losses	(1)	0
12 months or more, Fair Value	0	4
12 months or more, Gross Unrealized Losses	0	0
Total, Fair Value	36	6
Total, Gross Unrealized Losses	(1)	0
Debt Securities | Corporate Debt Securities
Investment [Line Items]
Less than 12 months, Fair Value	588	104
Less than 12 months, Gross Unrealized Losses	(28)	(2)
12 months or more, Fair Value	4	4
12 months or more, Gross Unrealized Losses	(1)	0
Total, Fair Value	592	108
Total, Gross Unrealized Losses	(29)	(2)
Debt Securities | U.S. Treasury And Other U.S. Government Corporations And Agencies | U.S. Treasury And Agency Obligations
Investment [Line Items]
Less than 12 months, Fair Value	298	56
Less than 12 months, Gross Unrealized Losses	(6)	0
12 months or more, Fair Value	6	2
12 months or more, Gross Unrealized Losses	0	0
Total, Fair Value	304	58
Total, Gross Unrealized Losses	(6)	0
Debt Securities | U.S. Treasury And Other U.S. Government Corporations And Agencies | Mortgage-Backed Securities
Investment [Line Items]
Less than 12 months, Fair Value	660	38
Less than 12 months, Gross Unrealized Losses	(26)	0
12 months or more, Fair Value	17	25
12 months or more, Gross Unrealized Losses	(1)	(1)
Total, Fair Value	677	63
Total, Gross Unrealized Losses	$ (27)	$ (1)