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Share-Based Compensation Weighted Average Assumptions and Fair Values Used for ESPP (Detail) - Employee Stock Purchase Plan - USD ($)
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Share based Compensation Arrangement by Share based Payment Award, Fair Value Assumptions, Method Used [Line Items]      
Expected term in years 6 months 6 months 6 months
Weighted average expected volatility of the Company's underlying common stock 25.90% 23.70% 22.30%
Risk-free interest rate 0.12% 0.08% 0.11%
Expected dividend yield $ 0 $ 0 $ 0
Weighted average fair value $ 29.27 $ 24.00 $ 15.35