Notional Amounts for Each Category of Derivative Activity (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivative
Balance at beginning of period	$ 799.2	$ 904.5	$ 1,052.0
Additions	553.7	136.5	92.1
Terminations	471.5	241.8	239.6
Foreign Currency	0.0
Balance at end of period	881.4	799.2	904.5
Interest Rate Swaps | Receive Variable/Pay Fixed
Derivative
Balance at beginning of period	0.0	48.0	105.5
Additions	0.0	0.0	0.0
Terminations	0.0	48.0	57.5
Foreign Currency	0.0
Balance at end of period	0.0	0.0	48.0
Interest Rate Swaps | Receive Fixed/Pay Fixed
Derivative
Balance at beginning of period	538.2	536.5	616.5
Additions	171.3	78.1	26.4
Terminations	98.4	76.4	106.4
Balance at end of period	611.1	538.2	536.5
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	0.0	0.0	0.0
Balance at end of period	250.0	250.0	250.0
Credit Default Swaps | Credit Default Swaps
Derivative
Balance at beginning of period	11.0	70.0	70.0
Additions	0.0	11.0	0.0
Terminations	0.0	70.0	0.0
Foreign Currency	0.4
Balance at end of period	11.4	11.0	70.0
Forwards
Derivative
Balance at beginning of period	0.0	0.0	10.0
Additions	382.4	47.4	65.7
Terminations	373.1	47.4	75.7
Foreign Currency	(0.4)
Balance at end of period	$ 8.9	$ 0.0	$ 0.0