Notional Amounts for Each Category of Derivative Activity (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Derivative
Balance at beginning of period	$ 1,470.3	$ 1,687.4
Additions	0.0	0.0
Terminations	24.0	8.9
Balance at ending of period	1,446.3	1,678.5
Swaps | Receive Variable/Pay Fixed
Derivative
Balance at beginning of period	150.0	150.0
Additions	0.0	0.0
Terminations	24.0	0.0
Balance at ending of period	126.0	150.0
Swaps | Receive Fixed/Pay Fixed
Derivative
Balance at beginning of period	650.3	840.4
Additions	0.0	0.0
Terminations	0.0	8.9
Balance at ending of period	650.3	831.5
Swaps | Receive Fixed/Pay Variable
Derivative
Balance at beginning of period	600.0	600.0
Additions	0.0	0.0
Terminations	0.0	0.0
Balance at ending of period	600.0	600.0
Swaps | Credit Default Swaps
Derivative
Balance at beginning of period	70.0	97.0
Additions	0.0	0.0
Terminations	0.0	0.0
Balance at ending of period	$ 70.0	$ 97.0