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Summary of Significant Accounting Policies - Additional Information (Detail)
 12 Months Ended
Oct. 30, 2017
USD ($)
Jun. 30, 2017
USD ($)
Feb. 08, 2017
USD ($)
Dec. 31, 2017
USD ($)
Segment
$ / shares
shares
Dec. 31, 2016
USD ($)
$ / shares
shares
Dec. 31, 2015
USD ($)
Summary Of Significant Accounting Policy [Line Items]            
Number of operating segments | Segment       1    
Interest costs incurred       $ 1,700,000 $ 1,300,000 $ 1,300,000
Interest costs capitalized       800,000 400,000  
Total capitalized payroll costs related to internal use software projects       15,800,000 8,800,000  
Amortization expense related to capitalized software costs       7,000,000 3,600,000 1,800,000
Goodwill impairment amount   $ 0   0 0 0
Impairment of intangible assets with finite lives       0 0 0
Impairment of long-lived assets       $ 0 0 0
Conversion fees, deferred revenue recognition period       10 years    
Advertising costs       $ 7,900,000 4,900,000 3,600,000
Sales taxes       $ 5,000,000 $ 4,300,000 3,700,000
Stock Repurchase Plan [Member]            
Summary Of Significant Accounting Policy [Line Items]            
Stock repurchase plan expiration date Oct. 30, 2019   Jan. 31, 2019      
Stock repurchase plan period 24 months          
Number of common stocks repurchased during the period | shares       1,238,577 1,112,261  
Stock repurchased, average costs per share | $ / shares       $ 72.45 $ 44.52  
Stock Repurchase Plan [Member] | Restricted Stock [Member]            
Summary Of Significant Accounting Policy [Line Items]            
Shares withheld to satisfy tax withholding obligations | shares       464,302 302,424  
Internal use Software [Member]            
Summary Of Significant Accounting Policy [Line Items]            
Capitalized and amortized period       3 years    
Customer Relationships [Member]            
Summary Of Significant Accounting Policy [Line Items]            
Capitalized and amortized period         6 months  
Expected period of client relationships       10 years    
Maximum [Member]            
Summary Of Significant Accounting Policy [Line Items]            
Interest costs capitalized           $ 100,000
Maximum [Member] | Stock Repurchase Plan [Member]            
Summary Of Significant Accounting Policy [Line Items]            
Stock repurchase plan, authorization amount $ 75,000,000   $ 50,000,000