Notional Amounts for Each Category of Derivative Activity (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Derivative
Balance at beginning of period	$ 1,013.7	$ 1,440.5	$ 1,052.0	$ 1,470.3
Additions	24.7	3.5	40.7	3.5
Terminations	127.3	357.1	181.6	386.9
Balance at ending of period	911.1	1,086.9	911.1	1,086.9
Swaps | Receive Variable/Pay Fixed
Derivative
Balance at beginning of period	102.0	126.0	105.5	150.0
Additions	0.0	3.5	0.0	3.5
Terminations	54.0	0.0	57.5	24.0
Balance at ending of period	48.0	129.5	48.0	129.5
Swaps | Receive Fixed/Pay Fixed
Derivative
Balance at beginning of period	591.7	644.5	616.5	650.3
Additions	0.0	0.0	0.0	0.0
Terminations	48.6	7.1	73.4	12.9
Balance at ending of period	543.1	637.4	543.1	637.4
Swaps | Receive Fixed/Pay Variable
Derivative
Balance at beginning of period	250.0	600.0	250.0	600.0
Additions	0.0	0.0	0.0	0.0
Terminations	0.0	350.0	0.0	350.0
Balance at ending of period	250.0	250.0	250.0	250.0
Swaps | Credit Default Swaps
Derivative
Balance at beginning of period	70.0	70.0	70.0	70.0
Additions	0.0	0.0	0.0	0.0
Terminations	0.0	0.0	0.0	0.0
Balance at ending of period	70.0	70.0	70.0	70.0
Forwards
Derivative
Balance at beginning of period	0.0	0.0	10.0	0.0
Additions	24.7	0.0	40.7	0.0
Terminations	24.7	0.0	50.7	0.0
Balance at ending of period	$ 0.0	$ 0.0	$ 0.0	$ 0.0