Derivative Financial Instruments (Fair Value of Hedged Items) (Details) - USD ($)	Jun. 30, 2019	Dec. 31, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Carrying value of the hedged items, available for sale debt securities	$ 3,255,800,000	$ 3,831,600,000
Cumulative hedge accounting basis adjustment, available for sale debt securities	15,800,000	99,400,000
Carrying value of the hedged items, senior notes and long-term subordinated debt	1,748,400,000	1,248,800,000
Cumulative hedge accounting basis adjustment, senior notes and long-term subordinated debt	119,700,000	29,300,000
Carrying Value of the Hedged Items	5,004,200,000	5,080,400,000
Cumulative Hedge Accounting Basis Adjustment	135,500,000	128,700,000
Hedged asset, fair value hedge, discontinued, cumulative increase (decrease)	2,000,000.0
Hedged liability, fair value hedge, discontinued, cumulative increase (decrease)	$ 0
Hedged asset (liability), fair value hedge, discontinued, cumulative increase (decrease), net		$ 0