Quarterly report pursuant to Section 13 or 15(d)

Stock-Based Compensation Plans - Components of Stock-Based Compensation Included in Selling, General and Administrative Expense in Condensed Consolidated Statements of Net and Comprehensive Income (Detail)

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Stock-Based Compensation Plans - Components of Stock-Based Compensation Included in Selling, General and Administrative Expense in Condensed Consolidated Statements of Net and Comprehensive Income (Detail) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share-based compensation expense - Independent contractors $ 1,474 $ 1,136 $ 4,712 $ 2,549
Allocated share-based compensation expense 2,168 1,417 6,750 3,275
Employee Stock Purchase Plan [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Allocated share-based compensation expense 56 45 184 68
Restricted Stock [Member] | Non-employee directors [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Allocated share-based compensation expense 91 61 228 137
Restricted Stock Units (RSUs) [Member] | Employees [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Allocated share-based compensation expense $ 547 $ 175 $ 1,626 $ 521