Other income (Schedule of Other Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Other income	$ 16.8	$ 14.2	$ 45.8	$ 42.6
Ancillary Product Profit Sharing [Member]
Disaggregation of Revenue [Line Items]
Other income	8.7	6.2	20.9	16.6
Remarketing Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	2.8	2.8	8.9	8.7
GPS-SID Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	1.4	2.3	5.3	9.0
Dealer Support Products And Services [Member]
Disaggregation of Revenue [Line Items]
Other income	1.0	1.2	3.4	3.7
Dealer Enrollment Fees [Member]
Disaggregation of Revenue [Line Items]
Other income	1.0	1.0	3.3	3.0
Other [Member]
Disaggregation of Revenue [Line Items]
Other income	$ 1.9	$ 0.7	$ 4.0	$ 1.6