Other income (Schedule of Other Income) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Other income	$ 7.0	$ 15.9	$ 35.2	$ 51.6
Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other income	4.1	8.8	22.3	27.3
Remarketing Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	1.1	3.0	5.7	9.5
Interest [Member]
Disaggregation of Revenue [Line Items]
Other income	0.4	2.0	2.6	6.7
Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	0.7	1.4	2.5	3.5
Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other income	0.5	0.6	1.5	2.0
GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0	0.0	0.0	1.9
Other [Member]
Disaggregation of Revenue [Line Items]
Other income	$ 0.2	$ 0.1	$ 0.6	$ 0.7