Other Income (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Other Income	$ 62.4	$ 57.4	$ 51.9
Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other Income	30.6	23.9	15.5
Remarketing Fees [Member]
Disaggregation of Revenue [Line Items]
Other Income	11.2	10.9	9.2
GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other Income	6.4	11.1	13.5
Interest Income [Member]
Disaggregation of Revenue [Line Items]
Other Income	5.0	2.2	1.7
Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other Income	4.3	3.9	5.0
Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other Income	4.1	4.8	4.7
Other [Member]
Disaggregation of Revenue [Line Items]
Other Income	0.8	$ 0.6	$ 2.3
Third Party Providers [Member]
Disaggregation of Revenue [Line Items]
Other Income	42.8
Third Party Providers [Member] | Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other Income	30.6
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	6.4
Third Party Providers [Member] | Interest Income [Member]
Disaggregation of Revenue [Line Items]
Other Income	5.0
Third Party Providers [Member] | Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other Income	0.0
Third Party Providers [Member] | Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other Income	0.0
Third Party Providers [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other Income	0.8
Dealers [Member]
Disaggregation of Revenue [Line Items]
Other Income	19.6
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	11.2
Dealers [Member] | GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other Income	0.0
Dealers [Member] | Interest Income [Member]
Disaggregation of Revenue [Line Items]
Other Income	0.0
Dealers [Member] | Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other Income	4.3
Dealers [Member] | Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other Income	4.1
Dealers [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other Income	0.0
Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Other Income	39.9
Transferred over Time [Member] | Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other Income	30.6
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] | Interest Income [Member]
Disaggregation of Revenue [Line Items]
Other Income	5.0
Transferred over Time [Member] | Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other Income	4.3
Transferred over Time [Member] | Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other Income	0.0
Transferred over Time [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other Income	0.0
Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Other Income	22.5
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	11.2
Transferred at Point in Time [Member] | GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other Income	6.4
Transferred at Point in Time [Member] | Interest Income [Member]
Disaggregation of Revenue [Line Items]
Other Income	0.0
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] | Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other Income	4.1
Transferred at Point in Time [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other Income	$ 0.8