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 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share-based compensation expense - Independent contractors $ 1,979 $ 926 $ 3,238 $ 1,413
Allocated share-based compensation expense 2,675 1,141 4,582 1,858
Employee Stock Purchase Plan [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Allocated share-based compensation expense 66 23 128 23
Restricted Stock [Member] | Non-employee directors [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Allocated share-based compensation expense 78 46 137 76
Restricted Stock Units (RSUs) [Member] | Employees [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Allocated share-based compensation expense $ 552 $ 146 $ 1,079 $ 346