Other Income (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Other income	$ 16.8	$ 14.2	$ 45.8	$ 42.6
Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other income	8.7	6.2	20.9	16.6
Remarketing Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	2.8	2.8	8.9	8.7
GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	1.4	2.3	5.3	9.0
Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other income	1.0	1.2	3.4	3.7
Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	1.0	1.0	3.3	3.0
Other [Member]
Disaggregation of Revenue [Line Items]
Other income	1.9	$ 0.7	4.0	$ 1.6
Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Other income	11.1		27.8
Transferred over Time [Member] | Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other income	8.7		20.9
Transferred over Time [Member] | Remarketing Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0		0.0
Transferred over Time [Member] | GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0		0.0
Transferred over Time [Member] | Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0		0.0
Transferred over Time [Member] | Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	1.0		3.3
Transferred over Time [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other income	1.4		3.6
Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Other income	5.7		18.0
Transferred at Point in Time [Member] | Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0		0.0
Transferred at Point in Time [Member] | Remarketing Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	2.8		8.9
Transferred at Point in Time [Member] | GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	1.4		5.3
Transferred at Point in Time [Member] | Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other income	1.0		3.4
Transferred at Point in Time [Member] | Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0		0.0
Transferred at Point in Time [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other income	0.5		0.4
Third Party Providers [Member]
Disaggregation of Revenue [Line Items]
Other income	12.0		30.2
Third Party Providers [Member] | Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other income	8.7		20.9
Third Party Providers [Member] | Remarketing Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0		0.0
Third Party Providers [Member] | GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	1.4		5.3
Third Party Providers [Member] | Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0		0.0
Third Party Providers [Member] | Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0		0.0
Third Party Providers [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other income	1.9		4.0
Dealers [Member]
Disaggregation of Revenue [Line Items]
Other income	4.8		15.6
Dealers [Member] | Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0		0.0
Dealers [Member] | Remarketing Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	2.8		8.9
Dealers [Member] | GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0		0.0
Dealers [Member] | Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other income	1.0		3.4
Dealers [Member] | Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	1.0		3.3
Dealers [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other income	$ 0.0		$ 0.0