Other income (Schedule of Other Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Other income	$ 15.9	$ 16.8	$ 51.6	$ 45.8
Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other income	8.8	8.7	27.3	20.9
Remarketing Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	3.0	2.8	9.5	8.9
Interest [Member]
Disaggregation of Revenue [Line Items]
Other income	2.0	1.4	6.7	3.6
Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	1.4	1.0	3.5	3.3
Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other income	0.6	1.0	2.0	3.4
GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	0.0	1.4	1.9	5.3
Other [Member]
Disaggregation of Revenue [Line Items]
Other income	$ 0.1	$ 0.5	$ 0.7	$ 0.4