Schedule of Gross Unrealized Losses and Fair Values of Securities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Investment [Line Items]
Less than 12 months, Fair Value	$ 619	$ 496
Less than 12 months, Gross Unrealized Losses	(7)	(8)
12 months or more, Fair Value	459	917
12 months or more, Gross Unrealized Losses	(24)	(33)
Total, Fair Value	1,078	1,413
Total, Gross Unrealized Losses	(31)	(41)
Debt Securities | Tax-exempt municipal securities
Investment [Line Items]
Less than 12 months, Fair Value	265	131
Less than 12 months, Gross Unrealized Losses	(3)	(1)
12 months or more, Fair Value	30	118
12 months or more, Gross Unrealized Losses	(1)	(2)
Total, Fair Value	295	249
Total, Gross Unrealized Losses	(4)	(3)
Debt Securities | Residential Mortgage-Backed Securities
Investment [Line Items]
Less than 12 months, Fair Value	0	1
Less than 12 months, Gross Unrealized Losses	0	0
12 months or more, Fair Value	5	4
12 months or more, Gross Unrealized Losses	0	0
Total, Fair Value	5	5
Total, Gross Unrealized Losses	0	0
Debt Securities | Commercial Mortgage-Backed Securities
Investment [Line Items]
Less than 12 months, Fair Value	50	31
Less than 12 months, Gross Unrealized Losses	(1)	(1)
12 months or more, Fair Value	262	267
12 months or more, Gross Unrealized Losses	(19)	(18)
Total, Fair Value	312	298
Total, Gross Unrealized Losses	(20)	(19)
Debt Securities | Asset-backed Securities
Investment [Line Items]
Less than 12 months, Fair Value	27	13
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	27	13
Total, Gross Unrealized Losses	0	0
Debt Securities | Corporate Debt Securities
Investment [Line Items]
Less than 12 months, Fair Value	173	219
Less than 12 months, Gross Unrealized Losses	(3)	(6)
12 months or more, Fair Value	40	128
12 months or more, Gross Unrealized Losses	(2)	(7)
Total, Fair Value	213	347
Total, Gross Unrealized Losses	(5)	(13)
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	47	79
Less than 12 months, Gross Unrealized Losses	0	0
12 months or more, Fair Value	24	80
12 months or more, Gross Unrealized Losses	0	(1)
Total, Fair Value	71	159
Total, Gross Unrealized Losses	0	(1)
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	57	22
Less than 12 months, Gross Unrealized Losses	0	0
12 months or more, Fair Value	98	320
12 months or more, Gross Unrealized Losses	(2)	(5)
Total, Fair Value	155	342
Total, Gross Unrealized Losses	$ (2)	$ (5)