Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net loss	$ (70,279)	$ (98,942)	$ (89,380)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Stock-based compensation expense	8,344	16,641	17,362
Depreciation and amortization expense	7,603	7,862	7,259
Non-cash operating lease cost	2,227
Non-cash interest expense	281		3
Accretion of discount on investments	(172)	(214)	(216)
Loss on disposal of property and equipment	103
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	3,257	(2,056)	31
Accounts receivable	(1,785)
Deferred revenue	(17,520)	13,476	(11,888)
Accounts payable	(1,460)	(353)	(856)
Operating lease liabilities	(4,211)
Accrued expenses and other liabilities	(2,908)	732	2,162
Net cash (used in) operating activities	(76,520)	(62,854)	(75,523)
Cash flows from investing activities:
Purchases of property and equipment	(1,002)	(1,937)	(4,676)
Purchases of investments	(46,420)	(21,832)	(97,934)
Sales and maturities of investments	16,904	136,087	158,312
Net cash (used in) provided by investing activities	(30,518)	112,318	55,702
Cash flows from financing activities:
Proceeds from public offering of common stock, net of commissions, underwriting discounts and offering costs	60,527
Proceeds from issuance of note payable	25,000
Proceeds from at the market equity offering	512
Payments of debt issuance costs	(425)
Proceeds from exercise of stock options	145	147	116
Proceeds from issuance of common stock and restricted common stock	176	61
Payments for repurchase of common stock for employee tax withholdings		(197)	(33)
Issuance of common stock under ESPP plan	296	257
Net cash provided by financing activities	86,231	268	83
Net (decrease) increase in cash, cash equivalents and restricted cash	(20,807)	49,732	(19,738)
Cash, cash equivalents and restricted cash at beginning of year	85,933	36,201	55,939
Cash, cash equivalents and restricted cash at end of year	65,126	85,933	36,201
Supplemental disclosure of cash flow information:
Cash paid for interest	221		1
Supplemental disclosure of non-cash investing and financing activities:
Property and equipment purchases included in accounts payable and accrued expenses	62	$ 157	$ 877
Reduction of operating lease assets and operating lease liabilities from operating lease modifications or reassessments	$ 154