Derivative Financial Instruments (Fair Value of Hedged Items) (Details) $ in Millions	Jun. 30, 2018 USD ($)
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Carrying value of the hedged items, available for sale debt securities	$ 4,038.6
Cumulative hedge accounting basis adjustment, available for sale debt securities	149.8
Carrying value of the hedged items, senior notes and long-term subordinated debt	1,049.4
Cumulative hedge accounting basis adjustment, senior notes and long-term subordinated debt	$ 1.9