Notional Amounts for Each Category of Derivative Activity (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Derivative
Balance at beginning of period	$ 1,446.3	$ 1,678.5	$ 1,470.3	$ 1,687.4
Additions	0.0	29.0	0.0	29.0
Terminations	5.8	68.8	29.8	77.7
Balance at ending of period	1,440.5	1,638.7	1,440.5	1,638.7
Swaps | Receive Variable/Pay Fixed
Derivative
Balance at beginning of period	126.0	150.0	150.0	150.0
Additions	0.0	0.0	0.0	0.0
Terminations	0.0	0.0	24.0	0.0
Balance at ending of period	126.0	150.0	126.0	150.0
Swaps | Receive Fixed/Pay Fixed
Derivative
Balance at beginning of period	650.3	831.5	650.3	840.4
Additions	0.0	0.0	0.0	0.0
Terminations	5.8	14.8	5.8	23.7
Balance at ending of period	644.5	816.7	644.5	816.7
Swaps | Receive Fixed/Pay Variable
Derivative
Balance at beginning of period	600.0	600.0	600.0	600.0
Additions	0.0	0.0	0.0	0.0
Terminations	0.0	0.0	0.0	0.0
Balance at ending of period	600.0	600.0	600.0	600.0
Swaps | Credit Default Swaps
Derivative
Balance at beginning of period	70.0	97.0	70.0	97.0
Additions	0.0	2.0	0.0	2.0
Terminations	0.0	27.0	0.0	27.0
Balance at ending of period	70.0	72.0	70.0	72.0
Forwards
Derivative
Balance at beginning of period	0.0	0.0	0.0	0.0
Additions	0.0	27.0	0.0	27.0
Terminations	0.0	27.0	0.0	27.0
Balance at ending of period	$ 0.0	$ 0.0	$ 0.0	$ 0.0