Notional Amounts for Each Category of Derivative Activity (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Derivative
Balance at beginning of period	$ 1,678.5	$ 1,469.5	$ 1,687.4	$ 1,477.4
Additions	29.0	0.0	29.0	10.0
Terminations	68.8	14.2	77.7	32.1
Balance at ending of period	1,638.7	1,455.3	1,638.7	1,455.3
Swaps | Receive Variable/Pay Fixed
Derivative
Balance at beginning of period	150.0	150.0	150.0	150.0
Additions	0.0	0.0	0.0	0.0
Terminations	0.0	0.0	0.0	0.0
Balance at ending of period	150.0	150.0	150.0	150.0
Swaps | Receive Fixed/Pay Fixed
Derivative
Balance at beginning of period	831.5	622.5	840.4	630.4
Additions	0.0	0.0	0.0	0.0
Terminations	14.8	14.2	23.7	22.1
Balance at ending of period	816.7	608.3	816.7	608.3
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	97.0	97.0	97.0	97.0
Additions	2.0	0.0	2.0	0.0
Terminations	27.0	0.0	27.0	0.0
Balance at ending of period	72.0	97.0	72.0	97.0
Forwards
Derivative
Balance at beginning of period	0.0	0.0	0.0	0.0
Additions	27.0	0.0	27.0	10.0
Terminations	27.0	0.0	27.0	10.0
Balance at ending of period	$ 0.0	$ 0.0	$ 0.0	$ 0.0