Assets and Liabilities Measured and Carried at Fair Value (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	$ 1,551,098	$ 1,551,098	$ 1,045,507
Contingent consideration	50,500	50,500	5,200
Total liabilities	50,500	50,500	5,200
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability	300
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability	45,000
Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	624,714	624,714	203,410
Government and Government Agency Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	892,614	892,614	809,159
Strategic Equity Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	32,769	32,769	31,937
Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	1,001	1,001	1,001
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	1,521,098	1,521,098	1,015,507
Level 1 [Member] | Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	624,714	624,714	203,410
Level 1 [Member] | Government and Government Agency Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	892,614	892,614	809,159
Level 1 [Member] | Strategic Equity Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	2,769	2,769	1,937
Level 1 [Member] | Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	1,001	1,001	1,001
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	30,000	30,000	30,000
Contingent consideration	50,500	50,500	5,200
Total liabilities	50,500	50,500	5,200
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability		300
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability		45,000
Level 3 [Member] | Strategic Equity Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	$ 30,000	$ 30,000	$ 30,000