Notional Amounts for Each Category of Derivative Activity (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Derivative
Balance at beginning of period	$ 846.3	$ 931.4	$ 744.4	$ 881.4
Additions	361.5	0.0	463.2	52.0
Terminations	288.7	0.0	288.7	1.3
Foreign Currency	0.0	0.0	0.2	(0.7)
Balance at end of period	919.1	931.4	919.1	931.4
Interest Rate Swaps | Receive Fixed/Pay Fixed
Derivative
Balance at beginning of period	747.0	663.1	720.8	611.1
Additions	61.5	0.0	87.7	52.0
Terminations	10.2	0.0	10.2	0.0
Foreign Currency	0.0	0.0	0.0	0.0
Balance at end of period	798.3	663.1	798.3	663.1
Interest Rate Swaps | Receive Fixed/Pay Variable
Derivative
Balance at beginning of period	0.0	250.0	0.0	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	0.0	250.0	0.0	250.0
Credit Default Swaps | Credit Default Swaps
Derivative
Balance at beginning of period	11.9	10.7	11.7	11.4
Additions	0.0	0.0	0.0	0.0
Terminations	0.0	0.0	0.0	0.0
Foreign Currency	0.0	0.0	0.2	(0.7)
Balance at end of period	11.9	10.7	11.9	10.7
Equity Contract | Total Return Swap
Derivative
Balance at beginning of period	75.5	0.0	0.0	0.0
Additions	5.3	0.0	80.8	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	80.8	0.0	80.8	0.0
Forwards
Derivative
Balance at beginning of period	11.9	7.6	11.9	8.9
Additions	294.7	0.0	294.7	0.0
Terminations	278.5	0.0	278.5	1.3
Foreign Currency	0.0	0.0	0.0	0.0
Balance at end of period	$ 28.1	$ 7.6	$ 28.1	$ 7.6