Debt (Schedule of Principal Debt Outstanding) (Details) - USD ($)		Sep. 30, 2018	Aug. 06, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Debt facility financing amount		$ 3,901,700,000		$ 3,093,000,000
Deferred Debt Issuance Costs		(23,600,000)		(20,900,000)
Unamortized debt discount		(1,200,000)		(1,300,000)
Carrying Amount		3,876,900,000		3,070,800,000
Other Assets [Member]
Debt Instrument [Line Items]
Deferred Debt Issuance Costs		(3,200,000)		(2,800,000)
Revolving Secured Line of Credit [Member]
Debt Instrument [Line Items]
Debt facility financing amount	[1]	0		13,900,000
Deferred Debt Issuance Costs	[1]	0		0
Unamortized debt discount	[1]	0		0
Carrying Amount	[1]	0		13,900,000
Secured financings [Member]
Debt Instrument [Line Items]
Debt facility financing amount		3,339,700,000		2,529,100,000
Deferred Debt Issuance Costs		(18,800,000)		(15,000,000)
Unamortized debt discount		0		0
Carrying Amount		3,320,900,000		2,514,100,000
Senior Notes [Member]
Debt Instrument [Line Items]
Debt facility financing amount		550,000,000		550,000,000
Deferred Debt Issuance Costs		(4,800,000)		(5,900,000)
Unamortized debt discount		(1,200,000)		(1,300,000)
Carrying Amount		544,000,000		$ 542,800,000
Mortgages [Member]
Debt Instrument [Line Items]
Debt facility financing amount		12,000,000	$ 12,000,000.0
Deferred Debt Issuance Costs		0
Unamortized debt discount		0
Carrying Amount		$ 12,000,000

[1] Warehouse facilities and Term ABS.