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Debt (Schedule of Additional Information Related to Debt Instruments) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Mar. 30, 2015
Debt Instrument [Line Items]
Secured Debt	$ 3,122.2		$ 3,122.2		$ 2,514.1
Restricted cash and cash equivalents	338.0	$ 313.9	338.0	$ 313.9	255.6	$ 224.7
Term ABS 2017-3 [Member]
Debt Instrument [Line Items]
Secured Debt	350.0		350.0		350.0
Loans pledged as collateral	442.7		442.7		482.6
Restricted cash and cash equivalents	$ 33.7		$ 33.7		$ 29.6
Senior notes yield to maturity	2.88%		2.88%		2.88%
Revolving Secured Line of Credit [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	$ 265.4	276.7	$ 265.4	276.7
Average outstanding balance	73.2	178.8	56.3	120.9
Warehouse Facility II [Member]
Debt Instrument [Line Items]
Secured Debt	0.0		0.0		$ 0.0
Maximum outstanding balance	201.0	263.4	201.0	263.4
Average outstanding balance	4.4	5.8	4.4	5.1
Loans pledged as collateral	0.0		0.0		0.0
Restricted cash and cash equivalents	$ 1.4		$ 1.4		$ 1.0
Senior notes yield to maturity	0.00%		0.00%		0.00%
Warehouse Facility IV [Member]
Debt Instrument [Line Items]
Secured Debt	$ 0.0		$ 0.0		$ 0.0
Maximum outstanding balance	99.0	12.0	99.0	12.0
Average outstanding balance	2.2	11.6	1.1	11.8
Loans pledged as collateral	0.0		0.0		0.0
Restricted cash and cash equivalents	$ 1.5		$ 1.5		$ 1.0
Senior notes yield to maturity	0.00%		0.00%		0.00%
Warehouse Facility V [Member]
Debt Instrument [Line Items]
Secured Debt	$ 0.0		$ 0.0		$ 0.0
Maximum outstanding balance	0.0	100.0	99.0	100.0
Average outstanding balance	0.0	28.9	1.1	15.6
Loans pledged as collateral	0.0		0.0		0.0
Restricted cash and cash equivalents	$ 1.2		$ 1.2		$ 1.0
Senior notes yield to maturity	0.00%		0.00%		0.00%
Warehouse Facility VI [Member]
Debt Instrument [Line Items]
Secured Debt	$ 0.0		$ 0.0		$ 0.0
Maximum outstanding balance	0.0	75.0	75.0	75.0
Average outstanding balance	0.0	28.4	0.8	14.8
Loans pledged as collateral	0.0		0.0
Restricted cash and cash equivalents	$ 0.1		$ 0.1		$ 0.0
Senior notes yield to maturity	0.00%		0.00%		0.00%
2023 Senior Notes [Member]
Debt Instrument [Line Items]
Senior notes yield to maturity	7.375%		7.375%		7.375%		7.50%
Warehouse Facility VII [Member]
Debt Instrument [Line Items]
Secured Debt	$ 0.0		$ 0.0		$ 0.0
Maximum outstanding balance	150.0	0.0	150.0	0.0
Average outstanding balance	26.4	$ 0.0	14.6	$ 0.0
Loans pledged as collateral	0.0		0.0		0.0
Restricted cash and cash equivalents	$ 1.2		$ 1.2		$ 1.0
Senior notes yield to maturity	0.00%		0.00%		0.00%
Term ABS 2018-1 [Member]
Debt Instrument [Line Items]
Secured Debt	$ 500.0		$ 500.0
Loans pledged as collateral	604.8		604.8
Restricted cash and cash equivalents	$ 46.4		$ 46.4
Senior notes yield to maturity	3.24%		3.24%