Schedule Of Gross Unrealized Losses And Fair Values Of Securities (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Investment [Line Items]
Less than 12 months, Fair Value	$ 226	$ 625
Less than 12 months, Gross Unrealized Losses	(4)	(11)
12 months or more, Fair Value	58	135
12 months or more, Gross Unrealized Losses	(2)	(6)
Total, Fair Value	284	760
Total, Gross Unrealized Losses	(6)	(17)
Debt Securities | Tax-exempt municipal securities
Investment [Line Items]
Less than 12 months, Fair Value	51	53
Less than 12 months, Gross Unrealized Losses	(1)	0
12 months or more, Fair Value	25	48
12 months or more, Gross Unrealized Losses	(1)	(2)
Total, Fair Value	76	101
Total, Gross Unrealized Losses	(2)	(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	10	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	10	14
Total, Gross Unrealized Losses	0	0
Debt Securities | Asset-backed Securities
Investment [Line Items]
Less than 12 months, Fair Value	0	16
Less than 12 months, Gross Unrealized Losses	0	0
12 months or more, Fair Value	7	4
12 months or more, Gross Unrealized Losses	0	0
Total, Fair Value	7	20
Total, Gross Unrealized Losses	0	0
Debt Securities | Corporate Debt Securities
Investment [Line Items]
Less than 12 months, Fair Value	62	355
Less than 12 months, Gross Unrealized Losses	(2)	(10)
12 months or more, Fair Value	6	41
12 months or more, Gross Unrealized Losses	0	(1)
Total, Fair Value	68	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	49	117
Less than 12 months, Gross Unrealized Losses	0	0
12 months or more, Fair Value	1	0
12 months or more, Gross Unrealized Losses	0	0
Total, Fair Value	50	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	54	67
Less than 12 months, Gross Unrealized Losses	(1)	(1)
12 months or more, Fair Value	15	18
12 months or more, Gross Unrealized Losses	0	(1)
Total, Fair Value	69	85
Total, Gross Unrealized Losses	$ (1)	$ (2)