Schedule of Gross Unrealized Losses and Fair Values of Securities (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Investment [Line Items]
Less than 12 months, Fair Value	$ 697	$ 2,905
Less than 12 months, Gross Unrealized Losses	(7)	(118)
12 months or more, Fair Value	1,064	141
12 months or more, Gross Unrealized Losses	(44)	(9)
Total, Fair Value	1,761	3,046
Total, Gross Unrealized Losses	(51)	(127)
Debt Securities | Tax-exempt municipal securities
Investment [Line Items]
Less than 12 months, Fair Value	136	693
Less than 12 months, Gross Unrealized Losses	0	(28)
12 months or more, Fair Value	136	57
12 months or more, Gross Unrealized Losses	(3)	(5)
Total, Fair Value	272	750
Total, Gross Unrealized Losses	(3)	(33)
Debt Securities | Residential Mortgage-Backed Securities
Investment [Line Items]
Less than 12 months, Fair Value	1	6
Less than 12 months, Gross Unrealized Losses	0	0
12 months or more, Fair Value	4	1
12 months or more, Gross Unrealized Losses	0	0
Total, Fair Value	5	7
Total, Gross Unrealized Losses	0	0
Debt Securities | Commercial Mortgage-Backed Securities
Investment [Line Items]
Less than 12 months, Fair Value	145	270
Less than 12 months, Gross Unrealized Losses	(3)	(8)
12 months or more, Fair Value	206	40
12 months or more, Gross Unrealized Losses	(14)	(1)
Total, Fair Value	351	310
Total, Gross Unrealized Losses	(17)	(9)
Debt Securities | Asset-backed Securities
Investment [Line Items]
Less than 12 months, Fair Value	0	35
Less than 12 months, Gross Unrealized Losses	0	(1)
12 months or more, Fair Value	16	0
12 months or more, Gross Unrealized Losses	0	0
Total, Fair Value	16	35
Total, Gross Unrealized Losses	0	(1)
Debt Securities | Corporate Debt Securities
Investment [Line Items]
Less than 12 months, Fair Value	209	594
Less than 12 months, Gross Unrealized Losses	(3)	(28)
12 months or more, Fair Value	141	17
12 months or more, Gross Unrealized Losses	(9)	(2)
Total, Fair Value	350	611
Total, Gross Unrealized Losses	(12)	(30)
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	59	231
Less than 12 months, Gross Unrealized Losses	0	(6)
12 months or more, Fair Value	86	5
12 months or more, Gross Unrealized Losses	(2)	0
Total, Fair Value	145	236
Total, Gross Unrealized Losses	(2)	(6)
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	147	1,076
Less than 12 months, Gross Unrealized Losses	(1)	(47)
12 months or more, Fair Value	475	21
12 months or more, Gross Unrealized Losses	(16)	(1)
Total, Fair Value	622	1,097
Total, Gross Unrealized Losses	$ (17)	$ (48)