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Debt (Schedule of Additional Information Related to Debt Instruments) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016
Revolving Secured Line of Credit [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Maximum outstanding balance | $ 130.5 | $ 92.1 | $ 276.7 | $ 186.4
Average outstanding balance | 35.7 | 13.3 | 92.2 | 30.6
Warehouse Facility II [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Maximum outstanding balance | 0.0 | 0.0 | 263.4 | 200.1
Average outstanding balance | 0.0 | 0.0 | 3.4 | 3.7
Warehouse Facility IV [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Maximum outstanding balance | 0.0 | 12.0 | 12.0 | 12.0
Average outstanding balance | 0.0 | 12.0 | 7.8 | 12.0
Warehouse Facility V [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Maximum outstanding balance | 0.0 | 0.0 | 100.0 | 100.0
Average outstanding balance | 0.0 | 0.0 | 10.4 | 1.4
Warehouse Facility VI [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Maximum outstanding balance | 0.0 | 0.0 | 75.0 | 49.9
Average outstanding balance | $ 0.0 | $ 0.0 | $ 9.8 | $ 4.0

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