Schedule of Gross Unrealized Losses and Fair Values of Securities (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Investment [Line Items]
Less than 12 months, Fair Value	$ 2,509	$ 496
Less than 12 months, Gross Unrealized Losses	(47)	(8)
12 months or more, Fair Value	468	917
12 months or more, Gross Unrealized Losses	(36)	(33)
Total, Fair Value	2,977	1,413
Total, Gross Unrealized Losses	(83)	(41)
Debt Securities | Tax-exempt municipal securities
Investment [Line Items]
Less than 12 months, Fair Value	518	131
Less than 12 months, Gross Unrealized Losses	(7)	(1)
12 months or more, Fair Value	24	118
12 months or more, Gross Unrealized Losses	(1)	(2)
Total, Fair Value	542	249
Total, Gross Unrealized Losses	(8)	(3)
Debt Securities | Residential Mortgage-Backed Securities
Investment [Line Items]
Less than 12 months, Fair Value	2	1
Less than 12 months, Gross Unrealized Losses	0	0
12 months or more, Fair Value	4	4
12 months or more, Gross Unrealized Losses	0	0
Total, Fair Value	6	5
Total, Gross Unrealized Losses	0	0
Debt Securities | Commercial Mortgage-Backed Securities
Investment [Line Items]
Less than 12 months, Fair Value	236	31
Less than 12 months, Gross Unrealized Losses	(3)	(1)
12 months or more, Fair Value	274	267
12 months or more, Gross Unrealized Losses	(25)	(18)
Total, Fair Value	510	298
Total, Gross Unrealized Losses	(28)	(19)
Debt Securities | Asset-backed securities
Investment [Line Items]
Less than 12 months, Fair Value	165	13
Less than 12 months, Gross Unrealized Losses	(1)	0
12 months or more, Fair Value	0	0
12 months or more, Gross Unrealized Losses	0	0
Total, Fair Value	165	13
Total, Gross Unrealized Losses	(1)	0
Debt Securities | Corporate debt securities
Investment [Line Items]
Less than 12 months, Fair Value	690	219
Less than 12 months, Gross Unrealized Losses	(33)	(6)
12 months or more, Fair Value	57	128
12 months or more, Gross Unrealized Losses	(8)	(7)
Total, Fair Value	747	347
Total, Gross Unrealized Losses	(41)	(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	35	79
Less than 12 months, Gross Unrealized Losses	0	0
12 months or more, Fair Value	19	80
12 months or more, Gross Unrealized Losses	0	(1)
Total, Fair Value	54	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	863	22
Less than 12 months, Gross Unrealized Losses	(3)	0
12 months or more, Fair Value	90	320
12 months or more, Gross Unrealized Losses	(2)	(5)
Total, Fair Value	953	342
Total, Gross Unrealized Losses	$ (5)	$ (5)