Fair Value Measurements - Fair Value Assets and Liabilities Measured on Recurring Basis (Detail) - Fair Value Measurements Recurring [Member] - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	$ 259,266	$ 531,006
Contingent Consideration [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities Fair Value Disclosure Recurring	46,500
Short-term Investments [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	183,834	308,506
Cash Equivalents [Member] | Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	50,959	177,577
Cash Equivalents [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	24,473	44,923
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	50,959	177,577
Fair Value, Inputs, Level 1 [Member] | Cash Equivalents [Member] | Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	50,959	177,577
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	208,307	353,429
Fair Value, Inputs, Level 2 [Member] | Short-term Investments [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	183,834	308,506
Fair Value, Inputs, Level 2 [Member] | Cash Equivalents [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	24,473	$ 44,923
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities Fair Value Disclosure Recurring	46,500
Fair Value, Inputs, Level 3 [Member] | Contingent Consideration [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities Fair Value Disclosure Recurring	$ 46,500