Fair Value Measurements - Summary of Financial Assets and Liabilities Measured and Recorded at Fair Value on Recurring Basis (Detail) - USD ($)	Dec. 31, 2020	Dec. 31, 2019	Aug. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 148,963,000	$ 139,045,000
Financial liabilities fair value disclosure	$ 0	0
2023 Notes | Convertible Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Stated interest percentage	1.25%		1.25%
Convertible debt, fair value disclosures	$ 501,800,000
2025 Notes | Convertible Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Stated interest percentage	2.25%		2.25%
Convertible debt, fair value disclosures	$ 373,300,000
U.S. Government Agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	83,587,000	36,979,000
Commercial Paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	34,766,000	19,966,000
Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	24,192,000	60,878,000
Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,999,000	12,242,000
Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,419,000	8,980,000
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign currency forward contracts designated as cash flow hedges	432,000
Total assets assets	301,965,000	245,826,000
Fair Value, Measurements, Recurring | U.S. Government Agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	83,587,000	36,979,000
Fair Value, Measurements, Recurring | Commercial Paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	34,766,000	19,966,000
Fair Value, Measurements, Recurring | Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	24,192,000	60,878,000
Fair Value, Measurements, Recurring | Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,999,000	12,242,000
Fair Value, Measurements, Recurring | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,419,000	8,980,000
Fair Value, Measurements, Recurring | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	152,570,000	106,781,000
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign currency forward contracts designated as cash flow hedges	0
Total assets assets	236,157,000	143,760,000
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | U.S. Government Agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	83,587,000	36,979,000
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Commercial Paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	152,570,000	106,781,000
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign currency forward contracts designated as cash flow hedges	432,000
Total assets assets	65,808,000	102,066,000
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | U.S. Government Agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Commercial Paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	34,766,000	19,966,000
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	24,192,000	60,878,000
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,999,000	12,242,000
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,419,000	8,980,000
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign currency forward contracts designated as cash flow hedges	0
Total assets assets	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | U.S. Government Agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Commercial Paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	$ 0	$ 0