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Acquisitions (Details)
$ in Thousands
3 Months Ended
Mar. 31, 2018
USD ($)
business
Mar. 31, 2017
USD ($)
Dec. 31, 2017
USD ($)
Business Acquisition [Line Items]      
Acquisition of businesses, net of cash acquired $ 36,214 $ 0  
Goodwill 5,078,370   $ 5,070,586
Dealer generated customer accounts and bulk account purchases $ 159,607 $ 143,688  
Series of Individually Immaterial Business Acquisitions      
Business Acquisition [Line Items]      
Number of businesses acquired | business 2    
Acquisition of businesses, net of cash acquired $ 36,000    
Goodwill 12,000    
Series of Individually Immaterial Business Acquisitions | Customer Relationships      
Business Acquisition [Line Items]      
Finite-lived intangible assets acquired $ 13,394