Schedule of Gross Unrealized Losses and Fair Values of Securities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Investment [Line Items]
Less than 12 months, Fair Value	$ 527	$ 625
Less than 12 months, Gross Unrealized Losses	(5)	(11)
12 months or more, Fair Value	66	135
12 months or more, Gross Unrealized Losses	(3)	(6)
Total, Fair Value	593	760
Total, Gross Unrealized Losses	(8)	(17)
Debt Securities | Tax-exempt municipal securities
Investment [Line Items]
Less than 12 months, Fair Value	233	53
Less than 12 months, Gross Unrealized Losses	(3)	0
12 months or more, Fair Value	27	48
12 months or more, Gross Unrealized Losses	(1)	(2)
Total, Fair Value	260	101
Total, Gross Unrealized Losses	(4)	(2)
Debt Securities | Residential Mortgage-Backed Securities
Investment [Line Items]
Less than 12 months, Fair Value	0	3
Less than 12 months, Gross Unrealized Losses	0	0
12 months or more, Fair Value	4	24
12 months or more, Gross Unrealized Losses	(1)	(2)
Total, Fair Value	4	27
Total, Gross Unrealized Losses	(1)	(2)
Debt Securities | Commercial Mortgage-Backed Securities
Investment [Line Items]
Less than 12 months, Fair Value	94	14
Less than 12 months, Gross Unrealized Losses	0	0
12 months or more, Fair Value	0	0
12 months or more, Gross Unrealized Losses	0	0
Total, Fair Value	94	14
Total, Gross Unrealized Losses	0	0
Debt Securities | Asset-backed Securities
Investment [Line Items]
Less than 12 months, Fair Value	2	16
Less than 12 months, Gross Unrealized Losses	0	0
12 months or more, Fair Value	4	4
12 months or more, Gross Unrealized Losses	0	0
Total, Fair Value	6	20
Total, Gross Unrealized Losses	0	0
Debt Securities | Corporate Debt Securities
Investment [Line Items]
Less than 12 months, Fair Value	104	355
Less than 12 months, Gross Unrealized Losses	(2)	(10)
12 months or more, Fair Value	4	41
12 months or more, Gross Unrealized Losses	0	(1)
Total, Fair Value	108	396
Total, Gross Unrealized Losses	(2)	(11)
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	56	117
Less than 12 months, Gross Unrealized Losses	0	0
12 months or more, Fair Value	2	0
12 months or more, Gross Unrealized Losses	0	0
Total, Fair Value	58	117
Total, Gross Unrealized Losses	0	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	38	67
Less than 12 months, Gross Unrealized Losses	0	(1)
12 months or more, Fair Value	25	18
12 months or more, Gross Unrealized Losses	(1)	(1)
Total, Fair Value	63	85
Total, Gross Unrealized Losses	$ (1)	$ (2)