Notional Amounts for Each Category of Derivative Activity (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Derivative
Balance at beginning of period	$ 1,052.0	$ 1,470.3
Additions	0.0	0.0
Terminations	30.4	24.0
Balance at ending of period	1,021.6	1,446.3
Swaps | Receive Variable/Pay Fixed
Derivative
Balance at beginning of period	105.5	150.0
Additions	0.0	0.0
Terminations	0.1	24.0
Balance at ending of period	105.4	126.0
Swaps | Receive Fixed/Pay Fixed
Derivative
Balance at beginning of period	616.5	650.3
Additions	0.0	0.0
Terminations	20.3	0.0
Balance at ending of period	596.2	650.3
Swaps | Receive Fixed/Pay Variable
Derivative
Balance at beginning of period	250.0	600.0
Additions	0.0	0.0
Terminations	0.0	0.0
Balance at ending of period	250.0	600.0
Swaps | Credit Default Swaps
Derivative
Balance at beginning of period	70.0	70.0
Additions	0.0	0.0
Terminations	0.0	0.0
Balance at ending of period	70.0	70.0
Forwards
Derivative
Balance at beginning of period	10.0	0.0
Additions	0.0	0.0
Terminations	10.0	0.0
Balance at ending of period	$ 0.0	$ 0.0