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Stock Based Compensation (Narrative) (Detail) - USD ($)
shares in Thousands, $ in Thousands
1 Months Ended 3 Months Ended 6 Months Ended 12 Months Ended
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Jun. 30, 2017
Dec. 15, 2015
Dec. 06, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Stock-based compensation expense       $ 4,514 $ 3,718      
Total unrecognized stock-based compensation cost   $ 9,750   $ 9,750        
2015 Equity Participation Plan (the "2015 Plan")                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Maximum number of shares of common stock that may be issued             4,250  
2010 Equity Participation Plan (as amended and restated in 2012, the "2010 Plan")                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Maximum number of shares of common stock that may be issued               5,250
Restricted stock | Employees                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Common stock granted, shares       424   277    
Restricted stock, vesting period       3 years   3 years    
Restricted stock | Non-employee directors                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Common stock granted, shares       27   22    
Restricted stock, vesting period       1 year   1 year    
Restricted stock units | Employees                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Common stock granted, shares           42    
Performance-vested restricted stock units                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Common stock granted, shares       203   160    
Restricted stock, vesting period       3 years   3 years    
Upper limit of target grant, shares       355   280    
Performance-vested restricted stock units, vesting percentage at the end of third year       100.00%   100.00%    
Restricted common stock and restricted stock units                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Stock-based compensation expense   $ 1,363 $ 2,048 $ 4,495 $ 3,707      
Restricted common stock and restricted stock units | Chief executive officer                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Stock-based compensation expense $ 2,017