Notional Amounts for Each Category of Derivative Activity (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative
Balance at beginning of period	$ 881.2	$ 1,021.6	$ 904.5	$ 1,052.0
Additions	27.1	16.0	73.8	16.0
Terminations	100.3	23.9	170.3	54.3
Balance at ending of period	808.0	1,013.7	808.0	1,013.7
Credit Default Swaps | Credit Default Swaps
Derivative
Balance at beginning of period	0.0	70.0	70.0	70.0
Additions	0.0	0.0	0.0	0.0
Terminations	0.0	0.0	70.0	0.0
Balance at ending of period	0.0	70.0	0.0	70.0
Forwards
Derivative
Balance at beginning of period	26.8	0.0	0.0	10.0
Additions	20.6	16.0	47.4	16.0
Terminations	26.8	16.0	26.8	26.0
Balance at ending of period	20.6	0.0	20.6	0.0
Receive Fixed/Pay Variable | Interest Rate Swaps
Derivative
Balance at beginning of period	250.0	250.0	250.0	250.0
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	250.0	250.0	250.0
Receive Fixed/Pay Fixed | Interest Rate Swaps
Derivative
Balance at beginning of period	556.4	596.2	536.5	616.5
Additions	6.5	0.0	26.4	0.0
Terminations	25.5	4.5	25.5	24.8
Balance at ending of period	537.4	591.7	537.4	591.7
Receive Variable/Pay Fixed | Interest Rate Swaps
Derivative
Balance at beginning of period	48.0	105.4	48.0	105.5
Additions	0.0	0.0	0.0	0.0
Terminations	48.0	3.4	48.0	3.5
Balance at ending of period	$ 0.0	$ 102.0	$ 0.0	$ 102.0