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Other income (Schedule of Other Income) (Details) - USD ($) $ in Millions | 3 Months Ended
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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

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- Definition Amount of revenue and income classified as other. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.4) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 + Details
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