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Stock-Based Compensation - Calculation of Fair Value of Each Employee Stock Purchase Grant Using the Black-Scholes Option-Pricing Model (Detail) - Employee Stock Purchase Plan ("ESPP") [Member]
9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Calculation of fair value of each employee stock purchase grant using the Black-Scholes option-pricing model    
Dividend yield 3.50% 4.20%
Risk-free interest rate 1.10% 0.60%
Expected volatility 34.00% 34.00%
Expected life (in years) 6 months 6 months