Schedule of Gross Unrealized Losses and Fair Values of Securities (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Investment [Line Items]
Less than 12 months, Fair Value	$ 2,429	$ 527
Less than 12 months, Gross Unrealized Losses	(92)	(5)
12 months or more, Fair Value	63	66
12 months or more, Gross Unrealized Losses	(3)	(3)
Total, Fair Value	2,492	593
Total, Gross Unrealized Losses	(95)	(8)
Debt Securities | Tax-exempt municipal securities
Investment [Line Items]
Less than 12 months, Fair Value	558	233
Less than 12 months, Gross Unrealized Losses	(24)	(3)
12 months or more, Fair Value	22	27
12 months or more, Gross Unrealized Losses	(1)	(1)
Total, Fair Value	580	260
Total, Gross Unrealized Losses	(25)	(4)
Debt Securities | Residential Mortgage-Backed Securities
Investment [Line Items]
Less than 12 months, Fair Value	7	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	9	4
Total, Gross Unrealized Losses	0	(1)
Debt Securities | Commercial Mortgage-Backed Securities
Investment [Line Items]
Less than 12 months, Fair Value	270	94
Less than 12 months, Gross Unrealized Losses	(8)	0
12 months or more, Fair Value	0	0
12 months or more, Gross Unrealized Losses	0	0
Total, Fair Value	270	94
Total, Gross Unrealized Losses	(8)	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	646	104
Less than 12 months, Gross Unrealized Losses	(29)	(2)
12 months or more, Fair Value	14	4
12 months or more, Gross Unrealized Losses	(1)	0
Total, Fair Value	660	108
Total, Gross Unrealized Losses	(30)	(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	241	56
Less than 12 months, Gross Unrealized Losses	(5)	0
12 months or more, Fair Value	2	2
12 months or more, Gross Unrealized Losses	0	0
Total, Fair Value	243	58
Total, Gross Unrealized Losses	(5)	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	671	38
Less than 12 months, Gross Unrealized Losses	(25)	0
12 months or more, Fair Value	23	25
12 months or more, Gross Unrealized Losses	(1)	(1)
Total, Fair Value	694	63
Total, Gross Unrealized Losses	$ (26)	$ (1)