Other income (Schedule of Other Income) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Other income	$ 14.0	$ 13.7	$ 29.0	$ 28.4
Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other income	5.8	5.3	12.2	10.4
Remarketing Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	3.0	2.8	6.1	5.9
GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	1.4	2.9	3.9	6.7
Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other income	1.2	1.2	2.4	2.5
Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	1.2	1.0	2.3	2.0
Other [Member]
Disaggregation of Revenue [Line Items]
Other income	$ 1.4	$ 0.5	$ 2.1	$ 0.9