Notional Amounts for Each Category of Derivative Activity (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative
Balance at beginning of period	$ 904.5	$ 1,052.0	$ 1,470.3
Additions	136.5	92.1	33.5
Terminations	241.8	239.6	451.8
Balance at ending of period	799.2	904.5	1,052.0
Interest Rate Swaps | Receive Variable/Pay Fixed
Derivative
Balance at beginning of period	48.0	105.5	150.0
Additions	0.0	0.0	3.5
Terminations	48.0	57.5	48.0
Balance at ending of period	0.0	48.0	105.5
Interest Rate Swaps | Receive Fixed/Pay Fixed
Derivative
Balance at beginning of period	536.5	616.5	650.3
Additions	78.1	26.4	0.0
Terminations	76.4	106.4	33.8
Balance at ending of period	538.2	536.5	616.5
Interest Rate Swaps | Receive Fixed/Pay Variable
Derivative
Balance at beginning of period	250.0	250.0	600.0
Additions	0.0	0.0	0.0
Terminations	0.0	0.0	350.0
Balance at ending of period	250.0	250.0	250.0
Credit Default Swaps | Credit Default Swaps
Derivative
Balance at beginning of period	70.0	70.0	70.0
Additions	11.0	0.0	0.0
Terminations	70.0	0.0	0.0
Balance at ending of period	11.0	70.0	70.0
Forwards
Derivative
Balance at beginning of period	0.0	10.0	0.0
Additions	47.4	65.7	30.0
Terminations	47.4	75.7	20.0
Balance at ending of period	$ 0.0	$ 0.0	$ 10.0