Other income (Schedule of Other Income) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Other income	$ 13.9	$ 16.0	$ 28.2	$ 35.7
Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other income	10.0	8.1	18.2	18.5
Remarketing Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	1.8	3.1	4.6	6.5
Interest [Member]
Disaggregation of Revenue [Line Items]
Other income	0.5	2.4	2.2	4.7
Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	0.9	1.1	1.8	2.1
Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other income	0.5	0.7	1.0	1.4
GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0	0.5	0.0	1.9
Other [Member]
Disaggregation of Revenue [Line Items]
Other income	$ 0.2	$ 0.1	$ 0.4	$ 0.6