Investment Securities (Details 2) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) securities	Dec. 31, 2016 USD ($) securities
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions [Abstract]
Number of securities, less than 12 months	34	37
Number of securities, 12 months or more | securities	13	6
Number of securities, total | securities	47	43
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Estimated Fair Value, Less than 12 months	$ 807	$ 451
Estimated Fair Value, 12 months or more	211	32
Estimated Fair Value, Total	1,018	483
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross Unrealized Losses, Less than 12 months	(6)	(19)
Gross Unrealized Losses, 12 months or more	(8)	(2)
Gross Unrealized Losses, Total	$ (14)	$ (21)
Ratio Of Fair Value To Amortized Cost Between Ninety And One Hundred Percent [Member]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions [Abstract]
Number of securities, less than 12 months | securities	34	33
Number of securities, 12 months or more | securities	13	6
Number of securities, total | securities	47	39
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Estimated Fair Value, Less than 12 months	$ 807	$ 411
Estimated Fair Value, 12 months or more	211	32
Estimated Fair Value, Total	1,018	443
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross Unrealized Losses, Less than 12 months	(6)	(13)
Gross Unrealized Losses, 12 months or more	(8)	(2)
Gross Unrealized Losses, Total	$ (14)	$ (15)
Ratio Of Fair Value To Amortized Cost Less Than Ninety Percent [Member]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions [Abstract]
Number of securities, less than 12 months | securities		4
Number of securities, 12 months or more | securities	0	0
Number of securities, total | securities	0	4
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Estimated Fair Value, Less than 12 months		$ 40
Estimated Fair Value, 12 months or more	$ 0	0
Estimated Fair Value, Total	0	40
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross Unrealized Losses, Less than 12 months	0	(6)
Gross Unrealized Losses, 12 months or more	0	0
Gross Unrealized Losses, Total	$ 0	$ (6)