Notional Amounts for Each Category of Derivative Activity (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Derivative
Balance at beginning of period	$ 744.4	$ 881.4	$ 799.2
Additions	1,540.5	120.0	553.7
Terminations	1,314.3	257.3	471.5
Foreign Currency	(0.1)	0.3	0.0
Balance at end of period	970.5	744.4	881.4
Interest Rate Swaps | Receive Fixed/Pay Fixed
Derivative
Balance at beginning of period	720.8	611.1	538.2
Additions	136.5	113.6	171.3
Terminations	29.3	3.9	98.4
Foreign Currency	0.0	0.0	0.0
Balance at end of period	828.0	720.8	611.1
Interest Rate Swaps | Receive Fixed/Pay Variable
Derivative
Balance at beginning of period	0.0	250.0	250.0
Additions	0.0	0.0	0.0
Terminations	0.0	250.0	0.0
Foreign Currency	0.0	0.0	0.0
Balance at end of period	0.0	0.0	250.0
Credit Default Swaps | Credit Default Swaps
Derivative
Balance at beginning of period	11.7	11.4	11.0
Additions	0.0	0.0	0.0
Terminations	0.0	0.0	0.0
Foreign Currency	(0.1)	0.3	0.4
Balance at end of period	11.6	11.7	11.4
Forwards
Derivative
Balance at beginning of period	11.9	8.9	0.0
Additions	340.5	6.4	382.4
Terminations	310.7	3.4	373.1
Foreign Currency	0.0	0.0	(0.4)
Balance at end of period	41.7	11.9	8.9
Equity Contract | Total Return Swap
Derivative
Balance at beginning of period	0.0	0.0	0.0
Additions	1,063.5	0.0	0.0
Terminations	974.3	0.0	0.0
Foreign Currency	0.0	0.0	0.0
Balance at end of period	$ 89.2	$ 0.0	$ 0.0