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Organization and Significant Accounting Policies - Accounting Policies (Details)
 3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Capitalized cost, amortization period under new and prior guidance   12 months
Revenue recognition, customer contract period, min 3 months  
Revenue recognition, customer contract period, max 36 months  
Sales commissions amortization period 3 years  
Accounting Standards Update 2014-09    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Capitalized cost, amortization period under new and prior guidance 3 years