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Stock-Based Compensation - Fair Value of Employees' Purchase Rights Estimated Using Black-Scholes Option-Pricing Model (Detail) (USD $)
3 Months Ended 12 Months Ended
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2012
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Employee stock purchase rights:                      
Expected life (years)                 6 months 6 months 6 months
Risk-free interest rate                 0.10% 0.10% 0.20%
Expected volatility                 27.70% 32.20% 31.90%
Expected annual dividends per share $ 0.16 $ 0.16 $ 0.16 $ 0.16 $ 0.16 $ 0.16 $ 0.15 $ 0.15 $ 0.64 $ 0.62 $ 0.60