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Stock-Based Awards - Summary of Option Activity (Detail) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]      
Options, beginning balance 1,368,247 1,544,597 5,330,070
Options, Granted 0 0 0
Options, Forfeitures/expirations 0 0 0
Options, Exercised   (176,350) (3,785,473)
Options, ending balance 1,158,828 1,368,247 1,544,597
Options, Exercised (209,419)    
Weighted Average Exercise Price of Options, beginning balance $ 2.82 $ 2.62 $ 1.61
Weighted Average Exercise Price of Options, Granted 0 0 0
Weighted Average Exercise Price of Options, Forfeitures or expirations 0 0 0
Weighted Average Exercise Price of Options, Exercised 3.17 1.11 1.19
Weighted Average Exercise Price of Options, ending balance $ 2.75 $ 2.82 $ 2.62