Weighted Average Assumptions and Fair Values Used for ESPP (Detail) (Employee Stock Purchase Plan, USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Employee Stock Purchase Plan
Share based Compensation Arrangement by Share based Payment Award, Fair Value Assumptions, Method Used [Line Items]
Weighted average expected term of options in years	6 months	6 months	6 months
Weighted average expected volatility of the Company's underlying common stock	43.50%	27.30%	36.70%
Risk-free interest rate	0.11%	0.15%	0.22%
Expected dividend yield	$ 0.0000	$ 0.0000	$ 0.0000
Weighted average fair value	$ 14.43	$ 13.02	$ 10.22