Investment Securities (Details 2) $ in Millions	Mar. 31, 2019 USD ($) securities	Dec. 31, 2018 USD ($) securities
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions [Abstract]
Number of securities, less than 12 months | securities	0	2
Number of securities, 12 months or more | securities	26	29
Number of securities, total | securities	26	31
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Estimated Fair Value, Less than 12 months	$ 0.0	$ 224.0
Estimated Fair Value, 12 months or more	1,050.0	873.0
Estimated Fair Value, Total	1,050.0	1,097.0
Available For Sale Securities Continuous Unrealized Aggregate Losses [Abstract]
Gross Unrealized Losses, Less than 12 Months	0.0	(2.0)
Gross Unrealized Losses, 12 months or more	(10.0)	(16.0)
Gross Unrealized Losses, Total	$ (10.0)	$ (18.0)
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	0	2
Number of securities, 12 months or more | securities	26	29
Number of securities, total | securities	26	31
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Estimated Fair Value, Less than 12 months	$ 0.0	$ 224.0
Estimated Fair Value, 12 months or more	1,050.0	873.0
Estimated Fair Value, Total	1,050.0	1,097.0
Available For Sale Securities Continuous Unrealized Aggregate Losses [Abstract]
Gross Unrealized Losses, Less than 12 Months	0.0	(2.0)
Gross Unrealized Losses, 12 months or more	(10.0)	(16.0)
Gross Unrealized Losses, Total	$ (10.0)	$ (18.0)