Annual report pursuant to Section 13 and 15(d)

Stock-Based Compensation Plans - Stock-Based Compensation Expense (Detail)

v3.8.0.1
Stock-Based Compensation Plans - Stock-Based Compensation Expense (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation expense - Independent contractors $ 4,870 $ 3,314 $ 4,159
Allocated share-based compensation expense 9,145 7,035 7,114
Employee Stock Purchase Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share-based compensation expense 128 169 285
Restricted Stock Awards [Member] | Non-Employee Directors [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share-based compensation expense 397 422 319
Restricted Stock Units [Member] | Employees [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share-based compensation expense $ 3,750 $ 3,130 $ 2,351