Notional Amounts for Each Category of Derivative Activity (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Derivative
Balance at beginning of period	$ 881.4	$ 799.2
Additions	52.0	55.6
Terminations	1.3	46.9
Foreign Currency	(0.7)	0.2
Balance at end of period	931.4	808.1
Interest Rate Swaps | Receive Fixed/Pay Fixed
Derivative
Balance at beginning of period	611.1	538.2
Additions	52.0	42.6
Terminations	0.0	46.9
Foreign Currency	0.0	0.0
Balance at end of period	663.1	533.9
Interest Rate Swaps | Receive Fixed/Pay Variable
Derivative
Balance at beginning of period	250.0	250.0
Additions	0.0	0.0
Terminations	0.0	0.0
Foreign Currency	0.0	0.0
Balance at end of period	250.0	250.0
Credit Default Swaps | Credit Default Swaps
Derivative
Balance at beginning of period	11.4	11.0
Additions	0.0	0.0
Terminations	0.0	0.0
Foreign Currency	(0.7)	0.2
Balance at end of period	10.7	11.2
Forwards
Derivative
Balance at beginning of period	8.9	0.0
Additions	0.0	13.0
Terminations	1.3	0.0
Foreign Currency	0.0	0.0
Balance at end of period	$ 7.6	$ 13.0