Fair Value Measurements - Fair Value Assets and Liabilities Measured on Recurring Basis (Detail) - Fair Value Measurements Recurring [Member] - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	$ 153,345	$ 1,269,128
Liabilities Fair Value Disclosure Recurring	465,900	320,100
Contingent Consideration [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities Fair Value Disclosure Recurring	465,900	320,100
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	107	625
Corporate Debt Securities [Member] | Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	51,032	814,946
Corporate Debt Securities [Member] | Long-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	2,540	108,288
Corporate Debt Securities [Member] | Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring		151,770
Foreign Certificates Of Deposit And Time [Member] | Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	4,847	53,786
Foreign Certificates Of Deposit And Time [Member] | Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	48,727	3,260
Mutual Fund | Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	46,092	44,441
US Government Agencies Debt Securities | Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring		25,000
US Government Agencies Debt Securities | Long-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring		67,012
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	107	625
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	107	625
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	153,238	1,268,503
Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member] | Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	51,032	814,946
Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member] | Long-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	2,540	108,288
Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member] | Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring		151,770
Fair Value, Inputs, Level 2 [Member] | Foreign Certificates Of Deposit And Time [Member] | Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	4,847	53,786
Fair Value, Inputs, Level 2 [Member] | Foreign Certificates Of Deposit And Time [Member] | Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	48,727	3,260
Fair Value, Inputs, Level 2 [Member] | Mutual Fund | Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	46,092	44,441
Fair Value, Inputs, Level 2 [Member] | US Government Agencies Debt Securities | Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring		25,000
Fair Value, Inputs, Level 2 [Member] | US Government Agencies Debt Securities | Long-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring		67,012
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities Fair Value Disclosure Recurring	465,900	320,100
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	$ 465,900	$ 320,100