Notional Amounts for Each Category of Derivative Activity (Detail) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2017	Jun. 30, 2016
Derivative
Balance at beginning of period	$ 1,021.6	$ 1,446.3	$ 1,470.3	$ 1,052.0	$ 1,470.3
Additions	16.0	0.0		16.0	0.0
Terminations	23.9	5.8		54.3	29.8
Balance at ending of period	1,013.7	1,440.5	1,446.3	1,013.7	1,440.5
Swaps | Receive Variable/Pay Fixed
Derivative
Balance at beginning of period	105.4	126.0	150.0	105.5	150.0
Additions	0.0	0.0		0.0	0.0
Terminations	3.4	0.0	24.0	3.5	24.0
Balance at ending of period	102.0	126.0	126.0	102.0	126.0
Swaps | Receive Fixed/Pay Fixed
Derivative
Balance at beginning of period	596.2	650.3	650.3	616.5	650.3
Additions	0.0	0.0		0.0	0.0
Terminations	4.5	5.8		24.8	5.8
Balance at ending of period	591.7	644.5	650.3	591.7	644.5
Swaps | Receive Fixed/Pay Variable
Derivative
Balance at beginning of period	250.0	600.0	600.0	250.0	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	250.0	600.0	600.0	250.0	600.0
Swaps | Credit Default Swaps
Derivative
Balance at beginning of period	70.0	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	70.0
Forwards
Derivative
Balance at beginning of period	0.0	0.0	0.0	10.0	0.0
Additions	16.0	0.0		16.0	0.0
Terminations	16.0	0.0		26.0	0.0
Balance at ending of period	$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ 0.0