Derivative Financial Instruments (Fair Value of Hedged Items) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Carrying value of the hedged items, available for sale debt securities	$ 3,759.5	$ 3,831.6
Cumulative hedge accounting basis adjustment, available for sale debt securities	60.5	99.4
Carrying value of the hedged items, senior notes and long-term subordinated debt	1,248.9	1,248.8
Cumulative hedge accounting basis adjustment, senior notes and long-term subordinated debt	55.1	29.3
Carrying Value of the Hedged Items	5,008.4	5,080.4
Cumulative Hedge Accounting Basis Adjustment	$ 115.6	$ 128.7