Schedule of Gross Unrealized Losses and Fair Values of Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Investment [Line Items]
Less than 12 months, Fair Value	$ 4,326	$ 496
Less than 12 months, Gross Unrealized Losses	(84)	(8)
12 months or more, Fair Value	484	917
12 months or more, Gross Unrealized Losses	(42)	(33)
Total, Fair Value	4,810	1,413
Total, Gross Unrealized Losses	(126)	(41)
Debt Securities | Tax-exempt municipal securities
Investment [Line Items]
Less than 12 months, Fair Value	843	131
Less than 12 months, Gross Unrealized Losses	(3)	(1)
12 months or more, Fair Value	52	118
12 months or more, Gross Unrealized Losses	(1)	(2)
Total, Fair Value	895	249
Total, Gross Unrealized Losses	(4)	(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	626	31
Less than 12 months, Gross Unrealized Losses	(13)	(1)
12 months or more, Fair Value	265	267
12 months or more, Gross Unrealized Losses	(28)	(18)
Total, Fair Value	891	298
Total, Gross Unrealized Losses	(41)	(19)
Debt Securities | Asset-backed securities
Investment [Line Items]
Less than 12 months, Fair Value	258	13
Less than 12 months, Gross Unrealized Losses	(2)	0
12 months or more, Fair Value	0	0
12 months or more, Gross Unrealized Losses	0	0
Total, Fair Value	258	13
Total, Gross Unrealized Losses	(2)	0
Debt Securities | Corporate debt securities
Investment [Line Items]
Less than 12 months, Fair Value	918	219
Less than 12 months, Gross Unrealized Losses	(45)	(6)
12 months or more, Fair Value	63	128
12 months or more, Gross Unrealized Losses	(10)	(7)
Total, Fair Value	981	347
Total, Gross Unrealized Losses	(55)	(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	195	79
Less than 12 months, Gross Unrealized Losses	(1)	0
12 months or more, Fair Value	14	80
12 months or more, Gross Unrealized Losses	0	(1)
Total, Fair Value	209	159
Total, Gross Unrealized Losses	(1)	(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	1,484	22
Less than 12 months, Gross Unrealized Losses	(20)	0
12 months or more, Fair Value	86	320
12 months or more, Gross Unrealized Losses	(3)	(5)
Total, Fair Value	1,570	342
Total, Gross Unrealized Losses	$ (23)	$ (5)