Investment Securities (Details 2) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) securities	Dec. 31, 2017 USD ($) securities
Available For Sale Securities Continuous Unrealized Loss Position Qualitative Disclosure [Abstract]
Number of securities, less than 12 months | securities	30	34
Number of securities, 12 months or more | securities	5	13
Number of securities, total | securities	35	47
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Estimated Fair Value, Less than 12 months	$ 955	$ 807
Estimated Fair Value, 12 months or more	173	211
Estimated Fair Value, Total	1,128	1,018
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross Unrealized Losses, Less than 12 months	(18)	(6)
Gross Unrealized Losses, 12 months or more	(7)	(8)
Gross Unrealized Losses, Total	$ (25)	$ (14)
Ratio Of Fair Value To Amortized Cost Between Ninety And One Hundred Percent [Member]
Available For Sale Securities Continuous Unrealized Loss Position Qualitative Disclosure [Abstract]
Number of securities, less than 12 months | securities	30	34
Number of securities, 12 months or more | securities	5	13
Number of securities, total | securities	35	47
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Estimated Fair Value, Less than 12 months	$ 955	$ 807
Estimated Fair Value, 12 months or more	173	211
Estimated Fair Value, Total	1,128	1,018
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross Unrealized Losses, Less than 12 months	(18)	(6)
Gross Unrealized Losses, 12 months or more	(7)	(8)
Gross Unrealized Losses, Total	$ (25)	$ (14)