Debt (Schedule of Additional Information Related to Debt Instruments) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Revolving Secured Line of Credit [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	$ 282.9	$ 265.4	$ 282.9	$ 265.4
Average outstanding balance	81.0	73.2	84.7	56.3
Warehouse Facility II [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	201.0	201.0	201.0	201.0
Average outstanding balance	88.4	4.4	90.7	4.4
Warehouse Facility IV [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	0.0	99.0	100.0	99.0
Average outstanding balance	0.0	2.2	2.2	1.1
Warehouse Facility V [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	0.0	0.0	0.0	99.0
Average outstanding balance	0.0	0.0	0.0	1.1
Warehouse Facility VI [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	0.0	0.0	0.0	75.0
Average outstanding balance	0.0	0.0	0.0	0.8
Warehouse Facility VII [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	25.0	150.0	101.5	150.0
Average outstanding balance	$ 1.9	$ 26.4	$ 2.1	$ 14.6