Debt (Schedule of Principal Debt Outstanding) (Details) - USD ($) $ in Millions		Sep. 30, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Principal Outstanding		$ 4,257.8	$ 3,842.5
Unamortized Debt Issuance Costs		(24.7)	(20.5)
Unamortized Discount		(0.9)	(1.1)
Carrying Amount		4,232.2	3,820.9
Other Assets [Member]
Debt Instrument [Line Items]
Unamortized Debt Issuance Costs		(3.5)	(2.9)
Revolving Secured Line of Credit [Member]
Debt Instrument [Line Items]
Principal Outstanding	[1]	52.2	171.9
Unamortized Debt Issuance Costs	[1]	0.0	0.0
Unamortized Discount	[1]	0.0	0.0
Carrying Amount	[1]	52.2	171.9
Secured financings [Member]
Debt Instrument [Line Items]
Principal Outstanding	[2]	3,244.1	3,108.7
Unamortized Debt Issuance Costs	[2]	(15.8)	(16.0)
Unamortized Discount	[2]	0.0	0.0
Carrying Amount	[2]	3,228.3	3,092.7
Senior Notes [Member]
Debt Instrument [Line Items]
Principal Outstanding		950.0	550.0
Unamortized Debt Issuance Costs		(8.9)	(4.5)
Unamortized Discount		(0.9)	(1.1)
Carrying Amount		940.2	544.4
Mortgages [Member]
Debt Instrument [Line Items]
Principal Outstanding		11.5	11.9
Unamortized Debt Issuance Costs		0.0	0.0
Unamortized Discount		0.0	0.0
Carrying Amount		$ 11.5	$ 11.9

[1] Excludes deferred debt issuance costs of $3.5 million and $2.9 million as of September 30, 2019 and December 31, 2018, respectively, which are included in other assets.

[2] Warehouse facilities and Term ABS.