Other Income (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Other income	$ 16.0	$ 14.0	$ 35.7	$ 29.0
Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other income	8.1	5.8	18.5	12.2
Remarketing Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	3.1	3.0	6.5	6.1
Interest [Member]
Disaggregation of Revenue [Line Items]
Other income	2.4	1.4	4.7	2.1
Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	1.1	1.2	2.1	2.3
Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other income	0.7	1.2	1.4	2.4
GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	0.5	1.4	1.9	3.9
Other [Member]
Disaggregation of Revenue [Line Items]
Other income	0.1	$ 0.0	0.6	$ 0.0
Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Other income	11.6		25.3
Transferred over Time [Member] | Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other income	8.1		18.5
Transferred over Time [Member] | Remarketing Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0		0.0
Transferred over Time [Member] | Interest [Member]
Disaggregation of Revenue [Line Items]
Other income	2.4		4.7
Transferred over Time [Member] | Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	1.1		2.1
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] | GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0		0.0
Transferred over Time [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0		0.0
Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Other income	4.4		10.4
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	3.1		6.5
Transferred at Point in Time [Member] | Interest [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0		0.0
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] | Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other income	0.7		1.4
Transferred at Point in Time [Member] | GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	0.5		1.9
Transferred at Point in Time [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other income	0.1		0.6
Third Party Providers [Member]
Disaggregation of Revenue [Line Items]
Other income	11.1		25.7
Third Party Providers [Member] | Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other income	8.1		18.5
Third Party Providers [Member] | Remarketing Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0		0.0
Third Party Providers [Member] | Interest [Member]
Disaggregation of Revenue [Line Items]
Other income	2.4		4.7
Third Party Providers [Member] | Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0		0.0
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] | GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	0.5		1.9
Third Party Providers [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other income	0.1		0.6
Dealers [Member]
Disaggregation of Revenue [Line Items]
Other income	4.9		10.0
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	3.1		6.5
Dealers [Member] | Interest [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0		0.0
Dealers [Member] | Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	1.1		2.1
Dealers [Member] | Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other income	0.7		1.4
Dealers [Member] | GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0		0.0
Dealers [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Other income	$ 0.0		$ 0.0