Debt (Schedule of Additional Information Related to Debt Instruments) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revolving Secured Line of Credit [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	$ 287.7	$ 193.3	$ 296.6	$ 282.9
Average outstanding balance	124.9	71.1	119.7	80.1
Warehouse Facility II [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	201.0	201.0	201.0	201.0
Average outstanding balance	161.5	126.7	127.8	102.8
Warehouse Facility IV [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	0.0	0.0	0.0	100.0
Average outstanding balance	0.0	0.0	0.0	1.5
Warehouse Facility V [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	75.0	35.0	75.0	35.0
Average outstanding balance	17.9	3.4	25.5	1.2
Warehouse Facility VI [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	50.0	0.0	50.0	0.0
Average outstanding balance	50.0	0.0	20.3	0.0
Warehouse Facility VII [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	125.0	50.0	125.0	101.5
Average outstanding balance	102.5	23.9	75.0	9.4
Warehouse Facility VIII [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	149.0	75.0	149.0	75.0
Average outstanding balance	$ 35.6	$ 12.7	$ 37.7	$ 12.7