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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, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Revolving Secured Line of Credit [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Maximum outstanding balance | $ 153.9 | $ 130.5 | $ 265.4 | $ 276.7
Average outstanding balance | 22.2 | 35.7 | 44.8 | 92.2
Warehouse Facility II [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Maximum outstanding balance | 201.0 | 0.0 | 201.0 | 263.4
Average outstanding balance | 4.4 | 0.0 | 4.4 | 3.4
Warehouse Facility IV [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Maximum outstanding balance | 0.0 | 0.0 | 99.0 | 12.0
Average outstanding balance | 0.0 | 0.0 | 0.7 | 7.8
Warehouse Facility V [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Maximum outstanding balance | 97.3 | 0.0 | 99.0 | 100.0
Average outstanding balance | 2.1 | 0.0 | 1.4 | 10.4
Warehouse Facility VI [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Maximum outstanding balance | 0.0 | 0.0 | 75.0 | 75.0
Average outstanding balance | 0.0 | 0.0 | 0.5 | 9.8
Warehouse Facility VII [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Maximum outstanding balance | 100.0 | 0.0 | 150.0 | 0.0
Average outstanding balance | $ 2.2 | $ 0.0 | $ 10.4 | $ 0.0

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