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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]
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Calculation of fair value of each employee stock purchase grant using the Black-Scholes option-pricing model    
Dividend yield 3.20% 3.80%
Risk-free interest rate 2.10% 0.80%
Expected volatility 33.00% 33.00%
Expected life (in years) 6 months 6 months