Notional Amounts for Each Category of Derivative Activity (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Derivative
Balance at beginning of period	$ 904.5	$ 1,052.0
Additions	46.7	0.0
Terminations	70.0	30.4
Balance at ending of period	881.2	1,021.6
Credit Risk Contract | Credit Default Swaps
Derivative
Balance at beginning of period	70.0	70.0
Additions	0.0	0.0
Terminations	70.0	0.0
Balance at ending of period	0.0	70.0
Forwards
Derivative
Balance at beginning of period	0.0	10.0
Additions	26.8	0.0
Terminations	0.0	10.0
Balance at ending of period	26.8	0.0
Receive Variable/Pay Fixed | Interest Rate Swaps
Derivative
Balance at beginning of period	48.0	105.5
Additions	0.0	0.0
Terminations	0.0	0.1
Balance at ending of period	48.0	105.4
Receive Fixed/Pay Fixed | Interest Rate Swaps
Derivative
Balance at beginning of period	536.5	616.5
Additions	19.9	0.0
Terminations	0.0	20.3
Balance at ending of period	556.4	596.2
Receive Fixed/Pay Variable | Interest Rate Swaps
Derivative
Balance at beginning of period	250.0	250.0
Additions	0.0	0.0
Terminations	0.0	0.0
Balance at ending of period	$ 250.0	$ 250.0