Annual report pursuant to Section 13 and 15(d)

Commitments and Contingencies - Narrative (Details)

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Commitments and Contingencies - Narrative (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Loss Contingencies [Line Items]      
Rent expense for operating leases $ 2,000,000.0 $ 1,800,000 $ 1,500,000
Minimum monthly purchase amount 40,000    
Minimum annual purchase amount $ 6,600,000    
Purchase commitment term 3 years    
Purchase commitment renewal term 3 years    
shareholder derivative claim [Member]      
Loss Contingencies [Line Items]      
Litigation Settlement, Amount $ 250,000    
Securities Class Action [Member]      
Loss Contingencies [Line Items]      
Litigation Settlement, Amount $ 1,750,000