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