Schedule of Gross Unrealized Losses and Fair Values of Securities (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Investment [Line Items]
Less than 12 months, Fair Value	$ 2,721	$ 496
Less than 12 months, Gross Unrealized Losses	(62)	(8)
12 months or more, Fair Value	462	917
12 months or more, Gross Unrealized Losses	(30)	(33)
Total, Fair Value	3,183	1,413
Total, Gross Unrealized Losses	(92)	(41)
Debt Securities | Tax-exempt municipal securities
Investment [Line Items]
Less than 12 months, Fair Value	685	131
Less than 12 months, Gross Unrealized Losses	(12)	(1)
12 months or more, Fair Value	38	118
12 months or more, Gross Unrealized Losses	(2)	(2)
Total, Fair Value	723	249
Total, Gross Unrealized Losses	(14)	(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	383	31
Less than 12 months, Gross Unrealized Losses	(8)	(1)
12 months or more, Fair Value	271	267
12 months or more, Gross Unrealized Losses	(22)	(18)
Total, Fair Value	654	298
Total, Gross Unrealized Losses	(30)	(19)
Debt Securities | Asset-backed securities
Investment [Line Items]
Less than 12 months, Fair Value	149	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	149	13
Total, Gross Unrealized Losses	0	0
Debt Securities | Corporate debt securities
Investment [Line Items]
Less than 12 months, Fair Value	708	219
Less than 12 months, Gross Unrealized Losses	(28)	(6)
12 months or more, Fair Value	38	128
12 months or more, Gross Unrealized Losses	(3)	(7)
Total, Fair Value	746	347
Total, Gross Unrealized Losses	(31)	(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	86	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	105	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	710	22
Less than 12 months, Gross Unrealized Losses	(14)	0
12 months or more, Fair Value	91	320
12 months or more, Gross Unrealized Losses	(3)	(5)
Total, Fair Value	801	342
Total, Gross Unrealized Losses	$ (17)	$ (5)