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Performance Awards - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]        
Unrecognized compensation expense $ 3.9   $ 3.9  
Performance Awards        
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]        
Compensation expense (benefit) (0.2) $ 0.3 0.6 $ 0.6
Unrecognized compensation expense $ 1.8 $ 1.7 $ 1.8 $ 1.7
2014 Equity Incentive Plan | Performance Awards        
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]        
Performance period     3 years