Notional Amounts for Each Category of Derivative Activity (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Derivative
Balance at beginning of period	$ 919.1	$ 931.4	$ 744.4	$ 881.4
Additions	29.7	43.1	492.9	95.1
Terminations	42.6	253.3	331.3	254.6
Foreign Currency	(0.3)	0.4	(0.1)	(0.3)
Balance at end of period	905.9	721.6	905.9	721.6
Interest Rate Swaps | Receive Fixed/Pay Fixed
Derivative
Balance at beginning of period	798.3	663.1	720.8	611.1
Additions	0.0	40.4	87.7	92.4
Terminations	10.5	3.3	20.7	3.3
Foreign Currency	0.0	0.0	0.0	0.0
Balance at end of period	787.8	700.2	787.8	700.2
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	250.0	0.0	250.0
Foreign Currency	0.0	0.0	0.0	0.0
Balance at end of period	0.0	0.0	0.0	0.0
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	(0.3)	0.4	(0.1)	(0.3)
Balance at end of period	11.6	11.1	11.6	11.1
Equity Contract | Total Return Swap
Derivative
Balance at beginning of period	80.8	0.0	0.0	0.0
Additions	2.2	0.0	83.0	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	83.0	0.0	83.0	0.0
Forwards
Derivative
Balance at beginning of period	28.1	7.6	11.9	8.9
Additions	27.5	2.7	322.2	2.7
Terminations	32.1	0.0	310.6	1.3
Foreign Currency	0.0	0.0	0.0	0.0
Balance at end of period	$ 23.5	$ 10.3	$ 23.5	$ 10.3