Debt (Summary of Term ABS Financings) (Details) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($)
Term ABS 2015-2 [Member]
Debt Instrument [Line Items]
Close Date	Aug. 20, 2015
Net Book Value of Loans Contributed at Closing	$ 375.5
Maturity Date	Aug. 15, 2017	[1],[2]
Term ABS 2016-1 [Member]
Debt Instrument [Line Items]
Close Date	Feb. 26, 2016
Net Book Value of Loans Contributed at Closing	$ 481.4
Maturity Date	Feb. 15, 2018	[1],[2]
Term ABS 2016-2 [Member]
Debt Instrument [Line Items]
Close Date	May 12, 2016
Net Book Value of Loans Contributed at Closing	$ 437.8
Maturity Date	May 15, 2018	[1],[2]
Term ABS 2016-3 [Member]
Debt Instrument [Line Items]
Close Date	Oct. 27, 2016
Net Book Value of Loans Contributed at Closing	$ 437.8
Maturity Date	Oct. 15, 2018	[1],[2]
Term ABS 2017-1 [Member]
Debt Instrument [Line Items]
Close Date	Feb. 23, 2017
Net Book Value of Loans Contributed at Closing	$ 437.8
Maturity Date	Feb. 15, 2019	[1],[2]
Term ABS 2017-2 [Member]
Debt Instrument [Line Items]
Close Date	Jun. 29, 2017
Net Book Value of Loans Contributed at Closing	$ 563.2
Maturity Date	Jun. 17, 2019	[1],[2]
Term ABS 2017-3 [Member]
Debt Instrument [Line Items]
Close Date	Oct. 26, 2017
Net Book Value of Loans Contributed at Closing	$ 437.6
Maturity Date	Oct. 15, 2019	[1],[2]
Term ABS 2018-1 [Member]
Debt Instrument [Line Items]
Close Date	Feb. 22, 2018
Net Book Value of Loans Contributed at Closing	$ 625.1
Maturity Date	Feb. 17, 2020	[1],[2]
Term ABS 2018-2 [Member]
Debt Instrument [Line Items]
Close Date	May 24, 2018
Net Book Value of Loans Contributed at Closing	$ 562.6
Maturity Date	May 15, 2020	[1],[2]
Term ABS 2018-3 [Member]
Debt Instrument [Line Items]
Close Date	Aug. 23, 2018
Net Book Value of Loans Contributed at Closing	$ 500.1
Maturity Date	Aug. 17, 2020	[1],[2]

[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.