Other income (Schedule of Other Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Other income	$ 15.0	$ 14.7
Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other income	6.4	5.1
Remarketing Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	3.1	3.1
GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	2.5	3.8
Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other income	1.2	1.3
Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	1.1	1.0
Other [Member]
Disaggregation of Revenue [Line Items]
Other income	$ 0.7	$ 0.4