Notional Amounts for Each Category of Derivative Activity (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Derivative
Balance at beginning of period	$ 1,687.4	$ 1,477.4	$ 1,432.8
Additions	96.0	318.1	291.0
Terminations	313.1	108.1	246.4
Balance at ending of period	1,470.3	1,687.4	1,477.4
Swaps | Receive Variable/Pay Fixed
Derivative
Balance at beginning of period	150.0	150.0	174.0
Additions	0.0	0.0	0.0
Terminations	0.0	0.0	24.0
Balance at ending of period	150.0	150.0	150.0
Swaps | Receive Fixed/Pay Fixed
Derivative
Balance at beginning of period	840.4	630.4	508.8
Additions	0.0	250.1	160.0
Terminations	190.1	40.1	38.4
Balance at ending of period	650.3	840.4	630.4
Swaps | Receive Fixed/Pay Variable
Derivative
Balance at beginning of period	600.0	600.0	750.0
Additions	0.0	0.0	0.0
Terminations	0.0	0.0	150.0
Balance at ending of period	600.0	600.0	600.0
Swaps | Credit Default Swaps
Derivative
Balance at beginning of period	97.0	97.0	0.0
Additions	2.0	0.0	97.0
Terminations	29.0	0.0	0.0
Balance at ending of period	70.0	97.0	97.0
Forwards
Derivative
Balance at beginning of period	0.0	0.0	0.0
Additions	94.0	68.0	24.0
Terminations	94.0	68.0	24.0
Balance at ending of period	0.0	0.0	0.0
Options Held
Derivative
Balance at beginning of period	0.0	0.0	0.0
Additions	0.0	0.0	10.0
Terminations	0.0	0.0	10.0
Balance at ending of period	$ 0.0	$ 0.0	$ 0.0