Other income (Schedule of Other Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Other income	$ 14.3	$ 19.7
Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other income	8.2	10.4
Remarketing Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	2.8	3.4
Interest [Member]
Disaggregation of Revenue [Line Items]
Other income	1.7	2.3
Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	0.9	1.0
Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other income	0.5	0.7
GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0	1.4
Other [Member]
Disaggregation of Revenue [Line Items]
Other income	$ 0.2	$ 0.5