Notional Amounts for Each Category of Derivative Activity (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Derivative
Balance at beginning of period	$ 1,687.4	$ 1,477.4
Additions	0	10.0
Terminations	8.9	17.9
Balance at ending of period	1,678.5	1,469.5
Swaps | Receive Variable/Pay Fixed
Derivative
Balance at beginning of period	150.0	150.0
Additions	0	0
Terminations	0	0
Balance at ending of period	150.0	150.0
Swaps | Receive Fixed/Pay Fixed
Derivative
Balance at beginning of period	840.4	630.4
Additions	0	0
Terminations	8.9	7.9
Balance at ending of period	831.5	622.5
Swaps | Receive Fixed/Pay Variable
Derivative
Balance at beginning of period	600.0	600.0
Additions	0	0
Terminations	0	0
Balance at ending of period	600.0	600.0
Swaps | Credit Default Swaps
Derivative
Balance at beginning of period	97.0	97.0
Additions	0	0
Terminations	0	0
Balance at ending of period	97.0	97.0
Forwards
Derivative
Balance at beginning of period	0	0
Additions	0	10.0
Terminations	0	10.0
Balance at ending of period	$ 0	$ 0