Schedule of Gross Unrealized Losses and Fair Values of Securities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Investment [Line Items]
Less than 12 months, Fair Value	$ 2,291	$ 2,905
Less than 12 months, Gross Unrealized Losses	(65)	(118)
12 months or more, Fair Value	326	141
12 months or more, Gross Unrealized Losses	(16)	(9)
Total, Fair Value	2,617	3,046
Total, Gross Unrealized Losses	(81)	(127)
Debt Securities | Tax-exempt municipal securities
Investment [Line Items]
Less than 12 months, Fair Value	413	693
Less than 12 months, Gross Unrealized Losses	(13)	(28)
12 months or more, Fair Value	115	57
12 months or more, Gross Unrealized Losses	(4)	(5)
Total, Fair Value	528	750
Total, Gross Unrealized Losses	(17)	(33)
Debt Securities | Residential Mortgage-Backed Securities
Investment [Line Items]
Less than 12 months, Fair Value	5	6
Less than 12 months, Gross Unrealized Losses	0	0
12 months or more, Fair Value	1	1
12 months or more, Gross Unrealized Losses	(1)	0
Total, Fair Value	6	7
Total, Gross Unrealized Losses	(1)	0
Debt Securities | Commercial Mortgage-Backed Securities
Investment [Line Items]
Less than 12 months, Fair Value	148	270
Less than 12 months, Gross Unrealized Losses	(3)	(8)
12 months or more, Fair Value	152	40
12 months or more, Gross Unrealized Losses	(8)	(1)
Total, Fair Value	300	310
Total, Gross Unrealized Losses	(11)	(9)
Debt Securities | Asset-backed Securities
Investment [Line Items]
Less than 12 months, Fair Value	35	35
Less than 12 months, Gross Unrealized Losses	0	(1)
12 months or more, Fair Value	0	0
12 months or more, Gross Unrealized Losses	0	0
Total, Fair Value	35	35
Total, Gross Unrealized Losses	0	(1)
Debt Securities | Corporate Debt Securities
Investment [Line Items]
Less than 12 months, Fair Value	333	594
Less than 12 months, Gross Unrealized Losses	(11)	(28)
12 months or more, Fair Value	35	17
12 months or more, Gross Unrealized Losses	(2)	(2)
Total, Fair Value	368	611
Total, Gross Unrealized Losses	(13)	(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	212	231
Less than 12 months, Gross Unrealized Losses	(4)	(6)
12 months or more, Fair Value	3	5
12 months or more, Gross Unrealized Losses	0	0
Total, Fair Value	215	236
Total, Gross Unrealized Losses	(4)	(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	1,145	1,076
Less than 12 months, Gross Unrealized Losses	(34)	(47)
12 months or more, Fair Value	20	21
12 months or more, Gross Unrealized Losses	(1)	(1)
Total, Fair Value	1,165	1,097
Total, Gross Unrealized Losses	$ (35)	$ (48)