Other Income (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Other income	$ 14.0	$ 13.7	$ 29.0	$ 28.4
Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other income	5.8	5.3	12.2	10.4
Remarketing Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	3.0	2.8	6.1	5.9
GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	1.4	2.9	3.9	6.7
Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other income	1.2	1.2	2.4	2.5
Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	1.2	1.0	2.3	2.0
Other [Member]
Disaggregation of Revenue [Line Items]
Other income	1.4	$ 0.5	2.1	$ 0.9
Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Other income	8.4		16.6
Transferred over Time [Member] | Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other income	5.8		12.2
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	1.2		2.3
Transferred over Time [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other income	1.4		2.1
Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Other income	5.6		12.4
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	3.0		6.1
Transferred at Point in Time [Member] | GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	1.4		3.9
Transferred at Point in Time [Member] | Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other income	1.2		2.4
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	0.0		0.0
Third Party Providers [Member]
Disaggregation of Revenue [Line Items]
Other income	8.6		18.2
Third Party Providers [Member] | Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other income	5.8		12.2
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		3.9
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	1.4		2.1
Dealers [Member]
Disaggregation of Revenue [Line Items]
Other income	5.4		10.8
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	3.0		6.1
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	1.2		2.4
Dealers [Member] | Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	1.2		2.3
Dealers [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other income	$ 0.0		$ 0.0