Notional Amounts for Each Category of Derivative Activity (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Derivative
Balance at beginning of period	$ 799.2	$ 904.5
Additions	55.6	46.7
Terminations	46.9	70.0
Foreign Currency	0.2
Balance at ending of period	808.1	881.2
Credit Risk Contract | Credit Default Swaps
Derivative
Balance at beginning of period	11.0	70.0
Additions	0.0	0.0
Terminations	0.0	70.0
Foreign Currency	0.2
Balance at ending of period	11.2	0.0
Forwards
Derivative
Balance at beginning of period	0.0	0.0
Additions	13.0	26.8
Terminations	0.0	0.0
Foreign Currency	0.0
Balance at ending of period	13.0	26.8
Receive Variable/Pay Fixed | Interest Rate Swaps
Derivative
Balance at beginning of period	0.0	48.0
Additions	0.0	0.0
Terminations	0.0	0.0
Foreign Currency	0.0
Balance at ending of period	0.0	48.0
Receive Fixed/Pay Fixed | Interest Rate Swaps
Derivative
Balance at beginning of period	538.2	536.5
Additions	42.6	19.9
Terminations	46.9	0.0
Foreign Currency	0.0
Balance at ending of period	533.9	556.4
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
Foreign Currency	0.0
Balance at ending of period	$ 250.0	$ 250.0