Derivative Financial Instruments (Fair Value of Hedged Items) (Details) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Carrying value of the hedged items, available for sale debt securities	$ 2,999,200,000	$ 2,981,000,000.0
Cumulative hedge accounting basis adjustment, available for sale debt securities	75,700,000	3,300,000
Carrying value of the hedged items, senior notes and long-term subordinated debt	1,748,500,000	1,748,500,000
Cumulative hedge accounting basis adjustment, senior notes and long-term subordinated debt	270,000,000.0	126,900,000
Carrying value of the hedged items, total	4,747,700,000	4,729,500,000
Cumulative hedge accounting basis adjustment, total	345,700,000	130,200,000
Hedged asset, fair value hedge, discontinued, cumulative increase (decrease)	1,600,000
Hedged liability, fair value hedge, discontinued, cumulative increase (decrease)	$ 0	$ 0