Changes in Stockholders' Equity - Summary of Changes in Stockholders' Equity (Detail) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Class Of Stock [Line Items]
Beginning Balance, Amount	$ 649,958	$ 244,641
Issuance of common stock under stock plans, net of shares withheld for employee taxes	1,320	1,162
Issuance of common stock in relation to common stock offering, net of offering costs incurred $365		342,630
Stock-based compensation	27,905	21,548
Cumulative translation adjustment	(1,077)	(304)
Unrealized gain on investments, net of income taxes of $0 and $113	583	426
Net loss	(17,716)	(11,100)
Ending Balance, Amount	660,973	599,003
Common Stock [Member]
Class Of Stock [Line Items]
Beginning Balance, Amount	$ 44	$ 40
Beginning Balance, Shares	42,955	39,300
Issuance of common stock under stock plans, net of shares withheld for employee taxes		$ 1
Issuance of common stock under stock plans, net of shares withheld for employee taxes, Shares	411	440
Issuance of common stock in relation to common stock offering, net of offering costs incurred $365		$ 2
Issuance of common stock in relation to common stock offering, net of offering costs incurred $365, Shares		2,151
Ending Balance, Amount	$ 44	$ 43
Ending Balance, Shares	43,366	41,891
Additional Paid-In Capital [Member]
Class Of Stock [Line Items]
Beginning Balance, Amount	$ 1,048,380	$ 589,708
Issuance of common stock under stock plans, net of shares withheld for employee taxes	1,320	1,161
Issuance of common stock in relation to common stock offering, net of offering costs incurred $365		342,628
Stock-based compensation	27,905	21,548
Ending Balance, Amount	1,077,605	955,045
Accumulated Other Comprehensive Income (Loss) [Member]
Class Of Stock [Line Items]
Beginning Balance, Amount	(336)	(723)
Cumulative translation adjustment	(1,077)	(304)
Unrealized gain on investments, net of income taxes of $0 and $113	583	426
Ending Balance, Amount	(830)	(601)
Accumulated Deficit [Member]
Class Of Stock [Line Items]
Beginning Balance, Amount	(398,130)	(344,384)
Net loss	(17,716)	(11,100)
Ending Balance, Amount	$ (415,846)	$ (355,484)