Debt Debt (Schedule of Principal Debt Outstanding) (Details) - USD ($) $ in Millions		Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Debt facility financing amount		$ 3,020.2	$ 2,622.1
Deferred Debt Issuance Costs		(18.0)	(16.8)
Unamortized debt discount		(1.4)	(1.6)
Carrying Amount		3,000.8	2,603.7
Revolving Secured Line of Credit [Member]
Debt Instrument [Line Items]
Debt facility financing amount	[1]	130.5	0.0
Deferred Debt Issuance Costs	[1]	0.0	0.0
Unamortized debt discount	[1]	0.0	0.0
Carrying Amount	[1]	130.5	0.0
Secured financings [Member]
Debt Instrument [Line Items]
Debt facility financing amount		2,339.7	2,072.1
Deferred Debt Issuance Costs		(11.8)	(9.7)
Unamortized debt discount		0.0	0.0
Carrying Amount		2,327.9	2,062.4
Senior Notes [Member]
Debt Instrument [Line Items]
Debt facility financing amount		550.0	550.0
Deferred Debt Issuance Costs		(6.2)	(7.1)
Unamortized debt discount		(1.4)	(1.6)
Carrying Amount		$ 542.4	$ 541.3

[1] Warehouse facilities and Term ABS.