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Stock-based Compensation - Summary of Activity for Non-voting Time-vested Restricted Stock Granted to Directors (Detail) - Non-Voting Time-Vested Restricted Stock Units [Member] - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]    
Share unit, Beginning Balance 202,560  
Share unit, Granted 480,065  
Share unit, Forfeited (10,211)  
Share unit, Vested and released to participants (137,703)  
Share unit, Ending Balance 534,711 202,560
Weighted Average Grant Fair Value, Beginning Balance $ 37.91  
Weighted Average Grants Date Fair Value, Granted 11.42 $ 20.07
Weighted Average Grants Date Fair Value, Forfeited 10.01  
Weighted Average Grants Date Fair Value, Vested and released to participants 41.76  
Weighted Average Grant Fair Value, Ending Balance $ 12.80 $ 37.91
Remaining Average Contractual Life, Ending 2 years 3 months 4 days 1 year 3 months
Aggregate Value $ 6,844 $ 7,679
Director [Member]    
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]    
Share unit, Beginning Balance 11,559  
Share unit, Granted 10,323  
Share unit, Forfeited 0  
Share unit, Vested and released to participants (2,202)  
Share unit, Ending Balance 19,680 11,559
Weighted Average Grant Fair Value, Beginning Balance $ 23.01  
Weighted Average Grants Date Fair Value, Granted 11.91  
Weighted Average Grants Date Fair Value, Forfeited 0.00  
Weighted Average Grants Date Fair Value, Vested and released to participants 44.04  
Weighted Average Grant Fair Value, Ending Balance $ 14.94 $ 23.01
Remaining Average Contractual Life, Ending 2 years 8 months 16 days 2 years 1 month 17 days
Aggregate Value $ 292 $ 266