Fair Value Hierarchy of Financial Assets Measured at Fair Value on Recurring Basis (Detail) (Fair Value, Measurements, Recurring, USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents (money market accounts and commercial paper investments)	$ 285,459	$ 325,652
Fair Value Assets And Liabilities Measured On Recurring Basis Net, Total	285,459	325,652
Cash Equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Valuation Technique (Approach)	Market	Market
Fair Value, Inputs, Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents (money market accounts and commercial paper investments)	285,459	325,652
Fair Value Assets And Liabilities Measured On Recurring Basis Net, Total	$ 285,459	$ 325,652