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Performance Awards - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]        
Unrecognized compensation expense $ 3.0   $ 3.0  
Performance Awards        
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]        
Compensation expense (benefit) 1.0 $ (0.3) 1.6 $ 0.4
Unrecognized compensation expense $ 1.6 $ 1.4 $ 1.6 $ 1.4
2014 Equity Incentive Plan | Performance Awards        
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]        
Performance period     3 years