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Stock-based Compensation - Summary of Activity for Non-voting Time-vested Restricted Stock (Detail) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Non-Voting Time-Vested Restricted Stock Units [Member]    
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]    
Share unit, Beginning Balance (shares) 534,711  
Share unit, Granted (shares) 366,055  
Share unit, Forfeited (shares) (6,755)  
Share unit, Vested and released to participants (shares) (28,723)  
Share unit, Ending Balance (shares) 865,288 534,711
Weighted Average Grant Fair Value, Beginning Balance (in USD per share) $ 12.80  
Weighted average grant date fair value, grants (in USD per share) 12.55 $ 11.42
Weighted Average Grants Date Fair Value, Forfeited (in USD per share) 19.86  
Weighted Average Grants Date Fair Value, Vested and released to participants (in USD per share) 29.45  
Weighted Average Grant Fair Value, Ending Balance (in USD per share) $ 11.73 $ 12.80
Remaining Average Contractual Life, Ending 1 year 10 months 17 days 2 years 3 months 4 days
Aggregate Value $ 10,146 $ 6,844
Non-Voting Performance Based Restricted Stock Units [Member]    
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]    
Share unit, Beginning Balance (shares) 818,150  
Share unit, Granted (shares) 448,012  
Share unit, Forfeited (shares) (165,397)  
Share unit, Vested and released to participants (shares) 0  
Share unit, Ending Balance (shares) 1,100,765 818,150
Weighted Average Grant Fair Value, Beginning Balance (in USD per share) $ 18.60  
Weighted average grant date fair value, grants (in USD per share) 12.54  
Weighted Average Grants Date Fair Value, Forfeited (in USD per share) 31.33  
Weighted Average Grants Date Fair Value, Vested and released to participants (in USD per share) 0.00  
Weighted Average Grant Fair Value, Ending Balance (in USD per share) $ 13.23 $ 18.60
Remaining Average Contractual Life, Ending 1 year 11 months 27 days 1 year 2 months 12 days
Aggregate Value $ 14,564 $ 15,218