Statement of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flow from operating activities
Net loss	$ (40,193)	$ (29,672)	$ (16,317)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	6,310	4,027	425
Depreciation	612	600	184
Amortization of premiums and discounts on marketable securities	696	452
Change in fair value of preferred stock tranche liability		9,750	1,949
Non-cash interest on convertible promissory notes payable			2
Expense related to shares issued in connection with services performed			250
In-kind research and development expenses	1,182	2,316
Other non-cash items	709	(277)	342
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(847)	(234)	(1,185)
Other non-current assets	7	14	(7)
Deferred revenue	(14,582)	52,666
Accounts payable	(62)	(942)	604
Accrued expenses	2,636	2,788	537
Other non-current liabilities		(189)	186
Lease incentive benefit	1,050		1,112
Net cash provided by (used in) operating activities	(42,482)	41,299	(11,918)
Cash flow from investing activities
Purchases of property and equipment	(5,029)	(1,030)	(2,988)
Change in restricted cash	(421)		(314)
Purchases of marketable securities	(112,350)	(220,399)
Proceeds from maturities of marketable securities	165,100	26,660
Net cash used in investing activities	47,300	(194,769)	(3,302)
Cash flow from financing activities
Proceeds from issuance of Series A redeemable convertible preferred stock and tranche rights, net of issuance costs		104,779	22,040
Proceeds from issuance of common stock and restricted stock		72,955
Proceeds from the exercise of stock options	514	10	80
Net cash provided by financing activities	514	177,744	22,120
Net increase in cash and cash equivalents	5,332	24,274	6,900
Cash and cash equivalents, beginning of period	31,309	7,035	135
Cash and cash equivalents, end of period	36,641	31,309	7,035
Supplemental Disclosure of Cash and non-cash Activities
Capital expenditures incurred but not yet paid	$ 242
Accretion of redeemable convertible preferred stock to redemption value		7,373	1,366
Exchange of promissory notes payable and accrued interest into Series A redeemable convertible preferred stock and tranche rights			$ 2,929
Conversion of redeemable convertible preferred stock to common stock		$ 150,187