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Summary of Significant Accounting Policies - Cash and Cash Equivalents and Accounts Receivable, net (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
customer
Dec. 31, 2016
USD ($)
Cash and Cash Equivalents    
Checks issued but not presented to banks | $ $ 22.8 $ 18.1
Accounts receivable | Credit concentration    
Accounts Receivable, net    
Number of customers | customer 1  
Customers individually representing greater than this percentage for disclosure 11.00% 10.00%