Derivative Financial Instruments Notional Amounts for Each Category of Derivative Activity (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Derivative
Balance at beginning of period	$ 808.1	$ 881.2	$ 799.2	$ 904.5
Additions	67.5	27.1	123.1	73.8
Terminations	55.2	100.3	102.1	170.3
Notional Amount Of Derivatives, Foreign Currency	(0.7)		(0.5)
Balance at ending of period	819.7	808.0	819.7	808.0
Interest Rate Swaps | Receive Variable/Pay Fixed
Derivative
Balance at beginning of period	0.0	48.0	0.0	48.0
Additions	0.0	0.0	0.0	0.0
Terminations	0.0	48.0	0.0	48.0
Notional Amount Of Derivatives, Foreign Currency	0.0		0.0
Balance at ending of period	0.0	0.0	0.0	0.0
Interest Rate Swaps | Receive Fixed/Pay Fixed
Derivative
Balance at beginning of period	533.9	556.4	538.2	536.5
Additions	57.0	6.5	99.6	26.4
Terminations	32.1	25.5	79.0	25.5
Notional Amount Of Derivatives, Foreign Currency	0.0		0.0
Balance at ending of period	558.8	537.4	558.8	537.4
Interest Rate Swaps | Receive Fixed/Pay Variable
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
Notional Amount Of Derivatives, Foreign Currency	0.0		0.0
Balance at ending of period	250.0	250.0	250.0	250.0
Credit Default Swaps | Credit Default Swaps
Derivative
Balance at beginning of period	11.2	0.0	11.0	70.0
Additions	0.0	0.0	0.0	0.0
Terminations	0.0	0.0	0.0	70.0
Notional Amount Of Derivatives, Foreign Currency	(0.3)		(0.1)
Balance at ending of period	10.9	0.0	10.9	0.0
Forwards
Derivative
Balance at beginning of period	13.0	26.8	0.0	0.0
Additions	10.5	20.6	23.5	47.4
Terminations	23.1	26.8	23.1	26.8
Notional Amount Of Derivatives, Foreign Currency	(0.4)		(0.4)
Balance at ending of period	$ 0.0	$ 20.6	$ 0.0	$ 20.6