Notional Amounts for Each Category of Derivative Activity (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Derivative
Balance at beginning of period	$ 1,440.5	$ 1,638.7	$ 1,470.3	$ 1,687.4
Additions	3.5	67.0	3.5	96.0
Terminations	357.1	5.5	386.9	83.2
Balance at ending of period	1,086.9	1,700.2	1,086.9	1,700.2
Swaps | Receive Variable/Pay Fixed
Derivative
Balance at beginning of period	126.0	150.0	150.0	150.0
Additions	3.5	0.0	3.5	0.0
Terminations	0.0	0.0	24.0	0.0
Balance at ending of period	129.5	150.0	129.5	150.0
Swaps | Receive Fixed/Pay Fixed
Derivative
Balance at beginning of period	644.5	816.7	650.3	840.4
Additions	0.0	0.0	0.0	0.0
Terminations	7.1	3.5	12.9	27.2
Balance at ending of period	637.4	813.2	637.4	813.2
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	350.0	0.0	350.0	0.0
Balance at ending of period	250.0	600.0	250.0	600.0
Swaps | Credit Default Swaps
Derivative
Balance at beginning of period	70.0	72.0	70.0	97.0
Additions	0.0	0.0	0.0	2.0
Terminations	0.0	2.0	0.0	29.0
Balance at ending of period	70.0	70.0	70.0	70.0
Forwards
Derivative
Balance at beginning of period	0.0	0.0	0.0	0.0
Additions	0.0	67.0	0.0	94.0
Terminations	0.0	0.0	0.0	27.0
Balance at ending of period	$ 0.0	$ 67.0	$ 0.0	$ 67.0