Notional Amounts for Each Category of Derivative Activity (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Derivative
Balance at beginning of period	$ 931.4	$ 819.7	$ 881.4	$ 799.2
Additions	43.1	380.9	95.1	504.0
Terminations	253.3	269.5	254.6	371.6
Foreign Currency	0.4	(0.3)	(0.3)	(0.8)
Balance at end of period	721.6	930.8	721.6	930.8
Interest Rate Swaps | Receive Fixed/Pay Fixed
Derivative
Balance at beginning of period	663.1	558.8	611.1	538.2
Additions	40.4	22.0	92.4	121.6
Terminations	3.3	13.5	3.3	92.5
Foreign Currency	0.0	0.0	0.0	0.0
Balance at end of period	700.2	567.3	700.2	567.3
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	250.0	0.0	250.0	0.0
Foreign Currency	0.0	0.0	0.0	0.0
Balance at end of period	0.0	250.0	0.0	250.0
Credit Default Swaps | Credit Default Swaps
Derivative
Balance at beginning of period	10.7	10.9	11.4	11.0
Additions	0.0	0.0	0.0	0.0
Terminations	0.0	0.0	0.0	0.0
Foreign Currency	0.4	(0.3)	(0.3)	(0.4)
Balance at end of period	11.1	10.6	11.1	10.6
Forwards
Derivative
Balance at beginning of period	7.6	0.0	8.9	0.0
Additions	2.7	358.9	2.7	382.4
Terminations	0.0	256.0	1.3	279.1
Foreign Currency	0.0	0.0	0.0	(0.4)
Balance at end of period	$ 10.3	$ 102.9	$ 10.3	$ 102.9