Securities (Fair Value and Gross Unrealized Losses on Available-for-Sale Securities) (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Investments, Unrealized Loss Position
Less than 12 months Fair Value	$ 1,335	$ 6,455
Less than 12 months Unrealized Losses	(10)	(70)
12 months or more Fair Value	9,679	15,111
12 months or more Unrealized Losses	(160)	(394)
Total Fair Value	11,014	21,566
Total Unrealized Losses	(170)	(464)
U.S. Treasury and federal agency
Investments, Unrealized Loss Position
Less than 12 months Fair Value	0	0
Less than 12 months Unrealized Losses	0	0
12 months or more Fair Value	74	97
12 months or more Unrealized Losses	0	(1)
Total Fair Value	74	97
Total Unrealized Losses	0	(1)
Agency mortgage-backed securities | Residential mortgage backed securities
Investments, Unrealized Loss Position
Less than 12 months Fair Value	539	3,235
Less than 12 months Unrealized Losses	(1)	(21)
12 months or more Fair Value	5,737	7,892
12 months or more Unrealized Losses	(101)	(221)
Total Fair Value	6,276	11,127
Total Unrealized Losses	(102)	(242)
Agency mortgage-backed securities | Commercial mortgage backed securities
Investments, Unrealized Loss Position
Less than 12 months Fair Value	358	2,022
Less than 12 months Unrealized Losses	(2)	(37)
12 months or more Fair Value	2,992	5,260
12 months or more Unrealized Losses	(48)	(127)
Total Fair Value	3,350	7,282
Total Unrealized Losses	(50)	(164)
Non-agency mortgage-backed securities | Commercial mortgage backed securities
Investments, Unrealized Loss Position
Less than 12 months Fair Value	0	884
Less than 12 months Unrealized Losses	0	(6)
12 months or more Fair Value	685	1,621
12 months or more Unrealized Losses	(6)	(41)
Total Fair Value	685	2,505
Total Unrealized Losses	(6)	(47)
Asset-backed securities and other debt securities
Investments, Unrealized Loss Position
Less than 12 months Fair Value	435	314
Less than 12 months Unrealized Losses	(7)	(6)
12 months or more Fair Value	191	241
12 months or more Unrealized Losses	(5)	(4)
Total Fair Value	626	555
Total Unrealized Losses	(12)	(10)
Obligations of states and political subdivisions
Investments, Unrealized Loss Position
Less than 12 months Fair Value	3
Less than 12 months Unrealized Losses	0
12 months or more Fair Value	0
12 months or more Unrealized Losses	0
Total Fair Value	3
Total Unrealized Losses	$ 0	$ 0