Other Income (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Other income	$ 15.0	$ 14.7
Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other income	6.4	5.1
Remarketing Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	3.1	3.1
GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	2.5	3.8
Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other income	1.2	1.3
Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	1.1	1.0
Other [Member]
Disaggregation of Revenue [Line Items]
Other income	0.7	$ 0.4
Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Other income	8.2
Transferred over Time [Member] | Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other income	6.4
Transferred over Time [Member] | Remarketing Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0
Transferred over Time [Member] | GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0
Transferred over Time [Member] | Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0
Transferred over Time [Member] | Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	1.1
Transferred over Time [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other income	0.7
Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Other income	6.8
Transferred at Point in Time [Member] | Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0
Transferred at Point in Time [Member] | Remarketing Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	3.1
Transferred at Point in Time [Member] | GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	2.5
Transferred at Point in Time [Member] | Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other income	1.2
Transferred at Point in Time [Member] | Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0
Transferred at Point in Time [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0
Third Party Providers [Member]
Disaggregation of Revenue [Line Items]
Other income	9.6
Third Party Providers [Member] | Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other income	6.4
Third Party Providers [Member] | Remarketing Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0
Third Party Providers [Member] | GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	2.5
Third Party Providers [Member] | Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0
Third Party Providers [Member] | Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0
Third Party Providers [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other income	0.7
Dealers [Member]
Disaggregation of Revenue [Line Items]
Other income	5.4
Dealers [Member] | Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0
Dealers [Member] | Remarketing Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	3.1
Dealers [Member] | GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0
Dealers [Member] | Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other income	1.2
Dealers [Member] | Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	1.1
Dealers [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other income	$ 0.0