Notional Amounts for Each Category of Derivative Activity (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative
Balance at beginning of period	$ 1,052.0	$ 1,470.3	$ 1,687.4
Additions	92.1	33.5	96.0
Terminations	239.6	451.8	313.1
Balance at ending of period	904.5	1,052.0	1,470.3
Swaps | Receive Variable/Pay Fixed
Derivative
Balance at beginning of period	105.5	150.0	150.0
Additions	0.0	3.5	0.0
Terminations	57.5	48.0	0.0
Balance at ending of period	48.0	105.5	150.0
Swaps | Receive Fixed/Pay Fixed
Derivative
Balance at beginning of period	616.5	650.3	840.4
Additions	26.4	0.0	0.0
Terminations	106.4	33.8	190.1
Balance at ending of period	536.5	616.5	650.3
Swaps | Receive Fixed/Pay Variable
Derivative
Balance at beginning of period	250.0	600.0	600.0
Additions	0.0	0.0	0.0
Terminations	0.0	350.0	0.0
Balance at ending of period	250.0	250.0	600.0
Swaps | Credit Default Swaps
Derivative
Balance at beginning of period	70.0	70.0	97.0
Additions	0.0	0.0	2.0
Terminations	0.0	0.0	29.0
Balance at ending of period	70.0	70.0	70.0
Forwards
Derivative
Balance at beginning of period	10.0	0.0	0.0
Additions	65.7	30.0	94.0
Terminations	75.7	20.0	94.0
Balance at ending of period	$ 0.0	$ 10.0	$ 0.0