Debt (Schedule of Additional Information Related to Debt Instruments) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Revolving Secured Line of Credit [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	$ 129.4	$ 296.6	$ 216.0	$ 296.6
Average outstanding balance	11.0	148.8	39.8	117.0
Warehouse Facility II [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	201.0	201.0	201.0	201.0
Average outstanding balance	4.4	201.0	23.5	110.7
Warehouse Facility IV [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	0.0	0.0	0.0	0.0
Average outstanding balance	0.0	0.0	0.0	0.0
Warehouse Facility V [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	0.0	75.0	0.0	75.0
Average outstanding balance	0.0	50.5	0.0	29.4
Warehouse Facility VI [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	0.0	0.0	0.0	0.0
Average outstanding balance	0.0	0.0	0.0	0.0
Warehouse Facility VII [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	0.0	125.0	0.0	125.0
Average outstanding balance	0.0	100.5	0.0	61.1
Warehouse Facility VIII [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	0.0	149.0	0.0	149.0
Average outstanding balance	$ 0.0	$ 69.9	$ 0.0	$ 38.7