Share-Based Compensation Weighted Average Assumptions and Fair Values Used for ESPP (Detail) (Employee Stock Purchase Plan, USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Employee Stock Purchase Plan
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	23.70%	22.30%	43.50%
Risk-free interest rate	0.08%	0.11%	0.11%
Expected dividend yield	$ 0	$ 0	$ 0
Weighted average fair value	$ 24.00	$ 15.35	$ 14.43