Notional Amounts for Each Category of Derivative Activity (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Derivative
Balance at beginning of period	$ 931.4	$ 808.1	$ 881.4	$ 799.2
Additions	0.0	67.5	52.0	123.1
Terminations	0.0	55.2	1.3	102.1
Foreign Currency	0.0	(0.7)	(0.7)	(0.5)
Balance at end of period	931.4	819.7	931.4	819.7
Interest Rate Swaps | Receive Fixed/Pay Fixed
Derivative
Balance at beginning of period	663.1	533.9	611.1	538.2
Additions	0.0	57.0	52.0	99.6
Terminations	0.0	32.1	0.0	79.0
Foreign Currency	0.0	0.0	0.0	0.0
Balance at end of period	663.1	558.8	663.1	558.8
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
Foreign Currency	0.0	0.0	0.0	0.0
Balance at end of period	250.0	250.0	250.0	250.0
Credit Default Swaps | Credit Default Swaps
Derivative
Balance at beginning of period	10.7	11.2	11.4	11.0
Additions	0.0	0.0	0.0	0.0
Terminations	0.0	0.0	0.0	0.0
Foreign Currency	0.0	(0.3)	(0.7)	(0.1)
Balance at end of period	10.7	10.9	10.7	10.9
Forwards
Derivative
Balance at beginning of period	7.6	13.0	8.9	0.0
Additions	0.0	10.5	0.0	23.5
Terminations	0.0	23.1	1.3	23.1
Foreign Currency	0.0	(0.4)	0.0	(0.4)
Balance at end of period	$ 7.6	$ 0.0	$ 7.6	$ 0.0