Other Income (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Other Income	$ 68.6	$ 62.4	$ 57.4
Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other Income	37.9	30.6	23.9
Remarketing Fees [Member]
Disaggregation of Revenue [Line Items]
Other Income	12.0	11.2	10.9
Interest Income [Member]
Disaggregation of Revenue [Line Items]
Other Income	8.5	5.0	2.2
Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other Income	4.7	4.3	3.9
Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other Income	2.5	4.1	4.8
GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other Income	1.9	6.4	11.1
Other [Member]
Disaggregation of Revenue [Line Items]
Other Income	1.1	$ 0.8	$ 0.6
Third Party Providers [Member]
Disaggregation of Revenue [Line Items]
Other Income	49.4
Third Party Providers [Member] | Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other Income	37.9
Third Party Providers [Member] | Remarketing Fees [Member]
Disaggregation of Revenue [Line Items]
Other Income	0.0
Third Party Providers [Member] | Interest Income [Member]
Disaggregation of Revenue [Line Items]
Other Income	8.5
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	1.1
Dealers [Member]
Disaggregation of Revenue [Line Items]
Other Income	19.2
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	12.0
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	4.7
Dealers [Member] | Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other Income	2.5
Dealers [Member] | GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other Income	0.0
Dealers [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other Income	0.0
Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Other Income	51.1
Transferred over Time [Member] | Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other Income	37.9
Transferred over Time [Member] | Remarketing 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	8.5
Transferred over Time [Member] | Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other Income	4.7
Transferred over Time [Member] | Dealer Support Products And Services [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] | Other [Member]
Disaggregation of Revenue [Line Items]
Other Income	0.0
Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Other Income	17.5
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	12.0
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	2.5
Transferred at Point in Time [Member] | GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other Income	1.9
Transferred at Point in Time [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other Income	$ 1.1