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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]
6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Calculation of fair value of each employee stock purchase grant using the Black-Scholes option-pricing model    
Dividend yield 2.50% 1.80%
Risk-free interest rate 0.30% 0.30%
Expected volatility 33.00% 39.00%
Expected life (in years) 6 months 6 months