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Debt (Schedule of Additional Information Related to Debt Instruments) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Mar. 30, 2015
Debt Instrument [Line Items]
Secured Debt	$ 2,880.2		$ 2,514.1
Restricted cash and cash equivalents	368.5	$ 316.7	255.6	$ 224.7
TermAssetBackedSecuredFinancingsTwoThousandSeventeenThree [Member]
Debt Instrument [Line Items]
Secured Debt	350.0		350.0
Loans pledged as collateral	473.6		482.6
Restricted cash and cash equivalents	$ 38.4		$ 29.6
Senior notes yield to maturity	2.88%		0.00%
Revolving Secured Line of Credit [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	$ 177.5	188.0
Average outstanding balance	39.1	62.4
Warehouse Facility II [Member]
Debt Instrument [Line Items]
Secured Debt	0.0		$ 0.0
Maximum outstanding balance	201.0	200.1
Average outstanding balance	4.5	4.5
Loans pledged as collateral	0.0		0.0
Restricted cash and cash equivalents	$ 1.4		$ 1.0
Senior notes yield to maturity	0.00%		0.00%
Warehouse Facility IV [Member]
Debt Instrument [Line Items]
Secured Debt	$ 0.0		$ 0.0
Maximum outstanding balance	0.0	12.0
Average outstanding balance	0.0	12.0
Loans pledged as collateral	0.0		0.0
Restricted cash and cash equivalents	$ 1.2		$ 1.0
Senior notes yield to maturity	0.00%		0.00%
Warehouse Facility V [Member]
Debt Instrument [Line Items]
Secured Debt	$ 0.0		$ 0.0
Maximum outstanding balance	99.0	100.0
Average outstanding balance	2.2	2.2
Loans pledged as collateral	0.0		0.0
Restricted cash and cash equivalents	$ 1.2		$ 1.0
Senior notes yield to maturity	0.00%		0.00%
Warehouse Facility VI [Member]
Debt Instrument [Line Items]
Secured Debt	$ 0.0		$ 0.0
Maximum outstanding balance	75.0	49.9
Average outstanding balance	1.7	1.1
Loans pledged as collateral	0.0
Restricted cash and cash equivalents	$ 0.1		$ 0.0
Senior notes yield to maturity	0.00%		0.00%
2023 Senior Notes [Member]
Debt Instrument [Line Items]
Senior notes yield to maturity	7.375%		7.375%		7.50%
Warehouse Facility VII [Member]
Debt Instrument [Line Items]
Secured Debt	$ 0.0
Maximum outstanding balance	125.0	0.0
Average outstanding balance	2.8	$ 0.0
Loans pledged as collateral	0.0
Restricted cash and cash equivalents	$ 1.1		$ 1.0
Senior notes yield to maturity	0.00%		0.00%
TermAssetBackedSecuredFinancingsTwoThousandEighteenOne [Member]
Debt Instrument [Line Items]
Secured Debt	$ 500.0
Loans pledged as collateral	629.5
Restricted cash and cash equivalents	$ 50.1
Senior notes yield to maturity	3.24%