Debt (Schedule of Principal Debt Outstanding) (Details) - USD ($) $ in Millions		Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Debt facility financing amount		$ 3,690.2	$ 3,093.0
Deferred Debt Issuance Costs		(23.2)	(20.9)
Unamortized debt discount		(1.2)	(1.3)
Carrying Amount		3,665.8	3,070.8
Other Assets [Member]
Debt Instrument [Line Items]
Deferred Debt Issuance Costs		(3.5)	(2.8)
Revolving Secured Line of Credit [Member]
Debt Instrument [Line Items]
Debt facility financing amount	[1]	0.0	13.9
Deferred Debt Issuance Costs	[1]	0.0	0.0
Unamortized debt discount	[1]	0.0	0.0
Carrying Amount	[1]	0.0	13.9
Secured financings [Member]
Debt Instrument [Line Items]
Debt facility financing amount		3,140.2	2,529.1
Deferred Debt Issuance Costs		(18.0)	(15.0)
Unamortized debt discount		0.0	0.0
Carrying Amount		3,122.2	2,514.1
Senior Notes [Member]
Debt Instrument [Line Items]
Debt facility financing amount		550.0	550.0
Deferred Debt Issuance Costs		(5.2)	(5.9)
Unamortized debt discount		(1.2)	(1.3)
Carrying Amount		$ 543.6	$ 542.8

[1] Warehouse facilities and Term ABS.