Debt (Schedule of Additional Information Related to Debt Instruments) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revolving Secured Line of Credit [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	$ 130.5	$ 92.1	$ 276.7	$ 186.4
Average outstanding balance	35.7	13.3	92.2	30.6
Warehouse Facility II [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	0.0	0.0	263.4	200.1
Average outstanding balance	0.0	0.0	3.4	3.7
Warehouse Facility IV [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	0.0	12.0	12.0	12.0
Average outstanding balance	0.0	12.0	7.8	12.0
Warehouse Facility V [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	0.0	0.0	100.0	100.0
Average outstanding balance	0.0	0.0	10.4	1.4
Warehouse Facility VI [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	0.0	0.0	75.0	49.9
Average outstanding balance	$ 0.0	$ 0.0	$ 9.8	$ 4.0