Other income (Schedule of Other Income) (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