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Stock-based Compensation - Schedule of ESPP Purchase Rights and Underlying Weighted Average Assumptions (Detail) - ESPP [Member] - $ / shares
3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Weighted average fair value per share $ 11.35 $ 11.42 $ 10.28 $ 11.12
Minimum [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Expected volatility 19.00% 25.00% 19.00% 25.00%
Risk-free interest rate 0.10% 0.10% 0.10% 0.10%
Expected term (in years) 6 months 6 months 6 months 6 months
Maximum [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Expected volatility 25.00% 27.00% 28.00% 28.00%
Risk-free interest rate 0.70% 0.50% 0.70% 0.50%
Expected term (in years) 2 years 2 years 2 years 2 years