Notional Amounts for Each Category of Derivative Activity (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative
Balance at beginning of period	$ 808.0	$ 1,013.7	$ 904.5	$ 1,052.0
Additions	43.1	24.7	116.9	40.7
Terminations	55.2	127.3	225.5	181.6
Balance at ending of period	795.9	911.1	795.9	911.1
Credit Default Swaps | Credit Default Swaps
Derivative
Balance at beginning of period	0.0	70.0	70.0	70.0
Additions	11.2	0.0	11.2	0.0
Terminations	0.0	0.0	70.0	0.0
Balance at ending of period	11.2	70.0	11.2	70.0
Forwards
Derivative
Balance at beginning of period	20.6	0.0	0.0	10.0
Additions	0.0	24.7	47.4	40.7
Terminations	20.6	24.7	47.4	50.7
Balance at ending of period	0.0	0.0	0.0	0.0
Receive Variable/Pay Fixed | Interest Rate Swaps
Derivative
Balance at beginning of period	0.0	102.0	48.0	105.5
Additions	0.0	0.0	0.0	0.0
Terminations	0.0	54.0	48.0	57.5
Balance at ending of period	0.0	48.0	0.0	48.0
Receive Fixed/Pay Fixed | Interest Rate Swaps
Derivative
Balance at beginning of period	537.4	591.7	536.5	616.5
Additions	31.9	0.0	58.3	0.0
Terminations	34.6	48.6	60.1	73.4
Balance at ending of period	534.7	543.1	534.7	543.1
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