Notional Amounts for Each Category of Derivative Activity (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Derivative
Balance at beginning of period	$ 1,638.7	$ 1,455.3	$ 1,687.4	$ 1,477.4
Additions	67.0	268.1	96.0	278.1
Terminations	5.5	21.8	83.2	53.9
Balance at ending of period	1,700.2	1,701.6	1,700.2	1,701.6
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	816.7	608.3	840.4	630.4
Additions	0.0	250.1	0.0	250.1
Terminations	3.5	3.8	27.2	25.9
Balance at ending of period	813.2	854.6	813.2	854.6
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	72.0	97.0	97.0	97.0
Additions	0.0	0.0	2.0	0.0
Terminations	2.0	0.0	29.0	0.0
Balance at ending of period	70.0	97.0	70.0	97.0
Forwards
Derivative
Balance at beginning of period	0.0	0.0	0.0	0.0
Additions	67.0	18.0	94.0	28.0
Terminations	0.0	18.0	27.0	28.0
Balance at ending of period	$ 67.0	$ 0.0	$ 67.0	$ 0.0