Derivative Financial Instruments Notional Amounts for Each Category of Derivative Activity (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Derivative
Balance at beginning of period	$ 819.7	$ 808.0	$ 799.2	$ 904.5
Additions	380.9	43.1	504.0	116.9
Terminations	269.5	55.2	371.6	225.5
Notional Amount Of Derivatives, Foreign Currency	(0.3)		(0.8)
Balance at ending of period	930.8	795.9	930.8	795.9
Interest Rate Swaps | Receive Variable/Pay Fixed
Derivative
Balance at beginning of period	0.0	0.0	0.0	48.0
Additions	0.0	0.0	0.0	0.0
Terminations	0.0	0.0	0.0	48.0
Notional Amount Of Derivatives, Foreign Currency	0.0		0.0
Balance at ending of period	0.0	0.0	0.0	0.0
Interest Rate Swaps | Receive Fixed/Pay Fixed
Derivative
Balance at beginning of period	558.8	537.4	538.2	536.5
Additions	22.0	31.9	121.6	58.3
Terminations	13.5	34.6	92.5	60.1
Notional Amount Of Derivatives, Foreign Currency	0.0		0.0
Balance at ending of period	567.3	534.7	567.3	534.7
Interest Rate Swaps | Receive Fixed/Pay Variable
Derivative
Balance at beginning of period	250.0	250.0	250.0	250.0
Additions	0.0	0.0	0.0	0.0
Terminations	0.0	0.0	0.0	0.0
Notional Amount Of Derivatives, Foreign Currency	0.0		0.0
Balance at ending of period	250.0	250.0	250.0	250.0
Credit Default Swaps | Credit Default Swaps
Derivative
Balance at beginning of period	10.9	0.0	11.0	70.0
Additions	0.0	11.2	0.0	11.2
Terminations	0.0	0.0	0.0	70.0
Notional Amount Of Derivatives, Foreign Currency	(0.3)		(0.4)
Balance at ending of period	10.6	11.2	10.6	11.2
Forwards
Derivative
Balance at beginning of period	0.0	20.6	0.0	0.0
Additions	358.9	0.0	382.4	47.4
Terminations	256.0	20.6	279.1	47.4
Notional Amount Of Derivatives, Foreign Currency	0.0		(0.4)
Balance at ending of period	$ 102.9	$ 0.0	$ 102.9	$ 0.0