Notional Amounts for Each Category of Derivative Activity (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Derivative
Balance at beginning of period	$ 744.4	$ 881.4
Additions	101.7	52.0
Terminations		1.3
Foreign Currency	0.2	(0.7)
Balance at end of period	846.3	931.4
Interest Rate Swaps | Receive Fixed/Pay Fixed
Derivative
Balance at beginning of period	720.8	611.1
Additions	26.2	52.0
Terminations		0.0
Foreign Currency	0.0	0.0
Balance at end of period	747.0	663.1
Interest Rate Swaps | Receive Fixed/Pay Variable
Derivative
Balance at beginning of period	0.0	250.0
Additions	0.0	0.0
Terminations		0.0
Foreign Currency	0.0	0.0
Balance at end of period	0.0	250.0
Credit Default Swaps | Credit Default Swaps
Derivative
Balance at beginning of period	11.7	11.4
Additions	0.0	0.0
Terminations		0.0
Foreign Currency	0.2	(0.7)
Balance at end of period	11.9	10.7
Forwards
Derivative
Balance at beginning of period	11.9	8.9
Additions	0.0	0.0
Terminations		1.3
Foreign Currency	0.0	0.0
Balance at end of period	11.9	7.6
Equity Contract | Total Return Swap
Derivative
Balance at beginning of period	0.0	0.0
Additions	75.5	0.0
Terminations		0.0
Foreign Currency	0.0	0.0
Balance at end of period	$ 75.5	$ 0.0