Schedule of Gross Unrealized Losses and Fair Values of Securities (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Investment [Line Items]
Less than 12 months, Fair Value	$ 1,648	$ 4,326
Less than 12 months, Gross Unrealized Losses	(24)	(84)
12 months or more, Fair Value	521	484
12 months or more, Gross Unrealized Losses	(40)	(42)
Total, Fair Value	2,169	4,810
Total, Gross Unrealized Losses	(64)	(126)
Debt Securities | Tax-exempt municipal securities
Investment [Line Items]
Less than 12 months, Fair Value	597	843
Less than 12 months, Gross Unrealized Losses	(2)	(3)
12 months or more, Fair Value	42	52
12 months or more, Gross Unrealized Losses	(1)	(1)
Total, Fair Value	639	895
Total, Gross Unrealized Losses	(3)	(4)
Debt Securities | Residential Mortgage-Backed Securities
Investment [Line Items]
Less than 12 months, Fair Value	2	2
Less than 12 months, Gross Unrealized Losses	0	0
12 months or more, Fair Value	3	4
12 months or more, Gross Unrealized Losses	0	0
Total, Fair Value	5	6
Total, Gross Unrealized Losses	0	0
Debt Securities | Commercial Mortgage-Backed Securities
Investment [Line Items]
Less than 12 months, Fair Value	276	626
Less than 12 months, Gross Unrealized Losses	(6)	(13)
12 months or more, Fair Value	249	265
12 months or more, Gross Unrealized Losses	(27)	(28)
Total, Fair Value	525	891
Total, Gross Unrealized Losses	(33)	(41)
Debt Securities | Asset-backed securities
Investment [Line Items]
Less than 12 months, Fair Value	146	258
Less than 12 months, Gross Unrealized Losses	0	(2)
12 months or more, Fair Value	0	0
12 months or more, Gross Unrealized Losses	0	0
Total, Fair Value	146	258
Total, Gross Unrealized Losses	0	(2)
Debt Securities | Corporate debt securities
Investment [Line Items]
Less than 12 months, Fair Value	374	918
Less than 12 months, Gross Unrealized Losses	(16)	(45)
12 months or more, Fair Value	97	63
12 months or more, Gross Unrealized Losses	(11)	(10)
Total, Fair Value	471	981
Total, Gross Unrealized Losses	(27)	(55)
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	76	195
Less than 12 months, Gross Unrealized Losses	0	(1)
12 months or more, Fair Value	4	14
12 months or more, Gross Unrealized Losses	0	0
Total, Fair Value	80	209
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	177	1,484
Less than 12 months, Gross Unrealized Losses	0	(20)
12 months or more, Fair Value	126	86
12 months or more, Gross Unrealized Losses	(1)	(3)
Total, Fair Value	303	1,570
Total, Gross Unrealized Losses	$ (1)	$ (23)