Debt (Schedule of Additional Information Related to Debt Instruments) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Revolving Secured Line of Credit [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	$ 216.0	$ 268.2
Average outstanding balance	68.9	85.2
Warehouse Facility II [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	201.0	201.0
Average outstanding balance	42.8	20.4
Warehouse Facility IV [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	0.0	0.0
Average outstanding balance	0.0	0.0
Warehouse Facility V [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	0.0	50.0
Average outstanding balance	0.0	8.2
Warehouse Facility VI [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	0.0	0.0
Average outstanding balance	0.0	0.0
Warehouse Facility VII [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	0.0	100.0
Average outstanding balance	0.0	21.7
Warehouse Facility VIII [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	0.0	49.0
Average outstanding balance	$ 0.0	$ 7.6