Notional Amounts for Each Category of Derivative Activity (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Derivative
Balance at beginning of period	$ 881.4	$ 799.2	$ 904.5
Additions	120.0	553.7	136.5
Terminations	257.3	471.5	241.8
Foreign Currency	0.3	0.0
Balance at end of period	744.4	881.4	799.2
Interest Rate Swaps | Receive Variable/Pay Fixed
Derivative
Balance at beginning of period	0.0	0.0	48.0
Additions	0.0	0.0	0.0
Terminations	0.0	0.0	48.0
Foreign Currency	0.0	0.0
Balance at end of period	0.0	0.0	0.0
Interest Rate Swaps | Receive Fixed/Pay Fixed
Derivative
Balance at beginning of period	611.1	538.2	536.5
Additions	113.6	171.3	78.1
Terminations	3.9	98.4	76.4
Balance at end of period	720.8	611.1	538.2
Interest Rate Swaps | Receive Fixed/Pay Variable
Derivative
Balance at beginning of period	250.0	250.0	250.0
Additions	0.0	0.0	0.0
Terminations	250.0	0.0	0.0
Balance at end of period	0.0	250.0	250.0
Credit Default Swaps | Credit Default Swaps
Derivative
Balance at beginning of period	11.4	11.0	70.0
Additions	0.0	0.0	11.0
Terminations	0.0	0.0	70.0
Foreign Currency	0.3	0.4
Balance at end of period	11.7	11.4	11.0
Forwards
Derivative
Balance at beginning of period	8.9	0.0	0.0
Additions	6.4	382.4	47.4
Terminations	3.4	373.1	47.4
Foreign Currency	0.0	(0.4)
Balance at end of period	$ 11.9	$ 8.9	$ 0.0