Derivative and Hedging Instruments (Schedule of Terms of Interest Rate Cap Agreements) (Details) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Debt facility financing amount		$ 3,469.9	$ 3,093.0
Interest rate caps, fair value of less than $0.1 million		0.0	0.0
Warehouse Facility II [Member]
Derivative [Line Items]
Debt facility financing amount	[1],[2]	400.0
Warehouse Facility II [Member] | 5.50% Cap Interest Rate [Member]
Derivative [Line Items]
Debt facility financing amount		400.0	400.0
Notional		$ 205.0	$ 205.0
Cap Interest Rate	[3]	5.50%	5.50%
Warehouse Facility IV [Member]
Derivative [Line Items]
Debt facility financing amount	[1],[2]	$ 100.0
Warehouse Facility IV [Member] | 5.50% Cap Interest Rate Original [Member]
Derivative [Line Items]
Notional		$ 75.0	$ 75.0
Cap Interest Rate	[3]	5.50%	5.50%
Warehouse Facility IV [Member] | Additional Interest Rate Cap [Member]
Derivative [Line Items]
Notional		$ 25.0	$ 25.0
Cap Interest Rate	[3]	6.50%	6.50%
Warehouse Facility IV [Member] | 5.50% Cap Interest Rate [Member]
Derivative [Line Items]
Debt facility financing amount		$ 100.0	$ 100.0
Notional		100.0	100.0
Warehouse Facility V [Member]
Derivative [Line Items]
Debt facility financing amount	[1],[2]	100.0
Warehouse Facility V [Member] | 5.50% Cap Interest Rate [Member]
Derivative [Line Items]
Debt facility financing amount		100.0	100.0
Notional		$ 75.0	$ 75.0
Cap Interest Rate	[3]	5.50%	5.50%
Term ABS 2016-1 [Member]
Derivative [Line Items]
Debt facility financing amount	[1],[2]	$ 385.0
Term ABS 2016-1 [Member] | 5.00% Cap Interest Rate [Member]
Derivative [Line Items]
Debt facility financing amount		355.2	$ 385.0
Notional		$ 352.9	$ 385.0
Cap Interest Rate	[3]	5.00%	5.00%
Warehouse Facility VII [Member]
Derivative [Line Items]
Debt facility financing amount	[1]	$ 150.0
Warehouse Facility VII [Member] | 5.00% Cap Interest Rate [Member]
Derivative [Line Items]
Debt facility financing amount		150.0	$ 150.0
Notional		$ 150.0	$ 150.0
Cap Interest Rate	[3]	5.50%	5.50%

[1] Financing made available only to a specified subsidiary of the Company.

[2] Represents the revolving maturity date. The outstanding balance will amortize after the revolving maturity date based on the cash flows of the pledged assets.

[3] Rate excludes the spread over the LIBOR rate.