Fair Value Measurements - Summary of Fair Value Measurements of Cash Equivalents, Available-for-Sale Marketable Securities and Free Share Asset/Liability (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	$ 293,350	$ 259,715
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	77,213	103,291
Commercial paper securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	180,812	177,224
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	71,114	63,870
U.S. government-sponsored entity debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	41,424	18,621
Fair value on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	77,213	103,291
Total marketable securities	293,350	259,715
Total cash equivalents and marketable securities and free shares asset	370,563	363,006
Fair value on recurring basis | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	77,213	103,291
Fair value on recurring basis | Commercial paper securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	180,812	177,224
Fair value on recurring basis | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	71,114	63,870
Fair value on recurring basis | U.S. government-sponsored entity debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	41,424	18,621
Fair value on recurring basis | Free shares asset
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities and free shares asset	239
Fair value on recurring basis | Free shares liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Free shares liability		154
Fair value on recurring basis | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	77,213	103,291
Total marketable securities	0	0
Total cash equivalents and marketable securities and free shares asset	77,213	103,291
Fair value on recurring basis | Level 1 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	77,213	103,291
Fair value on recurring basis | Level 1 | Commercial paper securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	0	0
Fair value on recurring basis | Level 1 | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	0	0
Fair value on recurring basis | Level 1 | U.S. government-sponsored entity debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	0	0
Fair value on recurring basis | Level 1 | Free shares asset
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities and free shares asset	0
Fair value on recurring basis | Level 1 | Free shares liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Free shares liability		0
Fair value on recurring basis | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	0	0
Total marketable securities	293,350	259,715
Total cash equivalents and marketable securities and free shares asset	293,350	259,715
Fair value on recurring basis | Level 2 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	0	0
Fair value on recurring basis | Level 2 | Commercial paper securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	180,812	177,224
Fair value on recurring basis | Level 2 | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	71,114	63,870
Fair value on recurring basis | Level 2 | U.S. government-sponsored entity debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	41,424	18,621
Fair value on recurring basis | Level 2 | Free shares asset
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities and free shares asset	0
Fair value on recurring basis | Level 2 | Free shares liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Free shares liability		0
Fair value on recurring basis | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	0	0
Total marketable securities	0	0
Total cash equivalents and marketable securities and free shares asset	0	0
Fair value on recurring basis | Level 3 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	0	0
Fair value on recurring basis | Level 3 | Commercial paper securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	0	0
Fair value on recurring basis | Level 3 | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	0	0
Fair value on recurring basis | Level 3 | U.S. government-sponsored entity debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	0	0
Fair value on recurring basis | Level 3 | Free shares asset
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities and free shares asset	$ 239
Fair value on recurring basis | Level 3 | Free shares liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Free shares liability		$ 154