Debt Debt (Schedule of Principal Debt Outstanding) (Details) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Principal Outstanding		$ 3,093.0	$ 2,622.1
Unamortized Debt Issuance Costs		20.9	16.8
Unamortized Discount		(1.3)	(1.6)
Debt Instrument, Carrying Amount		3,070.8	2,603.7
Other Assets [Member]
Debt Instrument [Line Items]
Unamortized Debt Issuance Costs	[1]	2.8	2.4
Revolving Secured Line Of Credit [Member]
Debt Instrument [Line Items]
Principal Outstanding	[2]	13.9	0.0
Unamortized Debt Issuance Costs	[2]	0.0	0.0
Unamortized Discount	[2]	0.0	0.0
Debt Instrument, Carrying Amount	[2]	13.9	0.0
Secured financings [Member]
Debt Instrument [Line Items]
Principal Outstanding	[1]	2,529.1	2,072.1
Unamortized Debt Issuance Costs	[1]	15.0	9.7
Unamortized Discount	[1]	0.0	0.0
Debt Instrument, Carrying Amount	[1]	2,514.1	2,062.4
Senior Notes [Member]
Debt Instrument [Line Items]
Principal Outstanding		550.0	550.0
Unamortized Debt Issuance Costs		5.9	7.1
Unamortized Discount		(1.3)	(1.6)
Debt Instrument, Carrying Amount		$ 542.8	$ 541.3

[1] Warehouse facilities and Term ABS financings.

[2] Excludes deferred debt issuance costs of $2.8 million and $2.4 million as of December 31, 2017 and December 31, 2016, respectively, which are included in other assets.