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Stock-based Compensation - Summary of Restricted Stock Units (Detail)
3 Months Ended
Mar. 31, 2018
$ / shares
shares
Time-Based Restricted Stock Units [Member]  
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]  
Number of Shares, Outstanding, Beginning Balance | shares 818,480
Number of Shares, Granted | shares 44,950
Number of Shares, Vested | shares (288,474)
Number of Shares, Forfeited or expired | shares (10,941)
Number of Shares, Outstanding, Ending Balance | shares 564,015
Weighted-Average Exercise Price, Outstanding, Beginning Balance | $ / shares $ 9.95
Weighted-Average Grant Date Fair Value, Granted | $ / shares 10.15
Weighted-Average Grant Date Fair Value, Vested | $ / shares 8.79
Weighted-Average Grant Date Fair Value, Forfeited or expired | $ / shares 10.28
Weighted-Average Exercise Price, Outstanding, Ending Balance | $ / shares $ 10.17
Performance-Based Restricted Stock Units [Member]  
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]  
Number of Shares, Outstanding, Beginning Balance | shares 444,734
Number of Shares, Vested | shares (168,068)
Number of Shares, Forfeited or expired | shares (38,288)
Number of Shares, Outstanding, Ending Balance | shares 238,378
Weighted-Average Exercise Price, Outstanding, Beginning Balance | $ / shares $ 9.48
Weighted-Average Grant Date Fair Value, Vested | $ / shares 9.16
Weighted-Average Grant Date Fair Value, Forfeited or expired | $ / shares 7.01
Weighted-Average Exercise Price, Outstanding, Ending Balance | $ / shares $ 9.98