Notional Amounts for Each Category of Derivative Activity (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Derivative
Balance at beginning of period	$ 1,470.3	$ 1,687.4	$ 1,477.4
Additions	33.5	96.0	318.1
Terminations	451.8	313.1	108.1
Balance at ending of period	1,052.0	1,470.3	1,687.4
Swaps | Receive Variable/Pay Fixed
Derivative
Balance at beginning of period	150.0	150.0	150.0
Additions	3.5	0.0	0.0
Terminations	48.0	0.0	0.0
Balance at ending of period	105.5	150.0	150.0
Swaps | Receive Fixed/Pay Fixed
Derivative
Balance at beginning of period	650.3	840.4	630.4
Additions	0.0	0.0	250.1
Terminations	33.8	190.1	40.1
Balance at ending of period	616.5	650.3	840.4
Swaps | Receive Fixed/Pay Variable
Derivative
Balance at beginning of period	600.0	600.0	600.0
Additions	0.0	0.0	0.0
Terminations	350.0	0.0	0.0
Balance at ending of period	250.0	600.0	600.0
Swaps | Credit Default Swaps
Derivative
Balance at beginning of period	70.0	97.0	97.0
Additions	0.0	2.0	0.0
Terminations	0.0	29.0	0.0
Balance at ending of period	70.0	70.0	97.0
Forwards
Derivative
Balance at beginning of period	0.0	0.0	0.0
Additions	30.0	94.0	68.0
Terminations	20.0	94.0	68.0
Balance at ending of period	$ 10.0	$ 0.0	$ 0.0