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Organization and Significant Accounting Policies - Revenue Recognition and Sales and Marketing Expenses (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]      
Revenue recognition, customer contract period, min 3 months    
Revenue recognition, customer contract period, max 36 months    
Sales commissions amortization period 12 months    
Advertising expense $ 2.7 $ 2.7 $ 2.8