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Stock-Based Compensation - Performance Stock Units Earned under Long-Term Incentive Plan (Detail) - Performance stock units
9 Months Ended
Sep. 30, 2017
shares
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]  
Number of Units Outstanding, Beginning Balance, Nonvested (in shares) 163,967
Number of Units, Granted (in shares) 64,208
Number of Units, Canceled (in shares) (6,807)
Number of Units, Accrued dividend equivalents (in shares) 1,437
Number of Units Outstanding, Ending Balance, Nonvested (in shares) 222,805