Debt (Schedule of Principal Debt Outstanding) - USD ($) $ in Millions		Mar. 31, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Principal Outstanding		$ 4,746.4	$ 4,635.1
Unamortized Debt Issuance Costs		(30.4)	(26.5)
Carrying Amount		4,716.0	4,608.6
Revolving Secured Line of Credit [Member]
Debt Instrument [Line Items]
Principal Outstanding	[1]	0.0	95.9
Unamortized Debt Issuance Costs	[1]	0.0	0.0
Carrying Amount	[1]	0.0	95.9
Secured financings [Member]
Debt Instrument [Line Items]
Principal Outstanding	[2]	3,936.1	3,728.7
Unamortized Debt Issuance Costs	[2]	(21.5)	(17.1)
Carrying Amount	[2]	3,914.6	3,711.6
Senior Notes [Member]
Debt Instrument [Line Items]
Principal Outstanding		800.0	800.0
Unamortized Debt Issuance Costs		(8.9)	(9.4)
Carrying Amount		791.1	790.6
Mortgages [Member]
Debt Instrument [Line Items]
Principal Outstanding		10.3	10.5
Unamortized Debt Issuance Costs		0.0	0.0
Carrying Amount		10.3	10.5
Other Assets [Member]
Debt Instrument [Line Items]
Unamortized Debt Issuance Costs		$ (2.9)	$ (3.2)

[1] Excludes deferred debt issuance costs of $2.9 million and $3.2 million as of March 31, 2021 and December 31, 2020, respectively, which are included in other assets.

[2] Warehouse facilities and Term ABS.