Fair Value Instruments - Summary of Cash, Cash Equivalents and Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 291,678	$ 187,514	$ 186,587	$ 103,670
Gross Unrealized Gains	38	10
Amortized Cost, Total	297,136	194,172
Aggregate Fair Value, Total	297,174	194,182
Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	291,678	187,514
Cash and cash equivalents, Aggregate Fair Value	291,678	187,514
Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	5,458	6,658
Gross Unrealized Gains	38	10
Aggregate Fair Value	5,496	6,668
Cash [Member] | Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	3	3
Cash and cash equivalents, Aggregate Fair Value	3	3
Money Market Instruments [Member] | Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	291,675	187,511
Cash and cash equivalents, Aggregate Fair Value	291,675	187,511
U.S. Government Agency Securities [Member] | Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	1,000	1,747
Gross Unrealized Gains	6	6
Aggregate Fair Value	1,006	1,753
Certificates of Deposit [Member] | Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	4,458	4,911
Gross Unrealized Gains	32	4
Aggregate Fair Value	4,490	4,915
Cash and Cash Equivalents [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Aggregate Fair Value, Total	291,678	187,514
Cash and Cash Equivalents [Member] | Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	291,678	187,514
Cash and Cash Equivalents [Member] | Cash [Member] | Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	3	3
Cash and Cash Equivalents [Member] | Money Market Instruments [Member] | Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	291,675	187,511
Short-term Marketable Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Aggregate Fair Value, Total	5,496	6,171
Short-term Marketable Securities [Member] | Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Aggregate Fair Value	5,496	6,171
Short-term Marketable Securities [Member] | U.S. Government Agency Securities [Member] | Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Aggregate Fair Value	1,006	1,753
Short-term Marketable Securities [Member] | Certificates of Deposit [Member] | Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Aggregate Fair Value	$ 4,490	4,418
Long-term Marketable Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Aggregate Fair Value, Total		497
Long-term Marketable Securities [Member] | Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Aggregate Fair Value		497
Long-term Marketable Securities [Member] | Certificates of Deposit [Member] | Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Aggregate Fair Value		$ 497