Statement of Cash Flows - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Cash flow from operating activities
Net loss	$ (3,833)	$ (29,672)	$ (16,317)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense		4,027	425
Depreciation		600	184
Amortization of premiums and discounts on marketable securities		452
Change in fair value of preferred stock tranche liability		9,750	1,949
Non-cash interest on convertible promissory notes payable	67		2
Expense related to shares issued in connection with services performed			250
Deferred rent		277	(342)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets		(234)	(1,185)
Other non-current assets	(14)	14	(7)
Deferred revenue		54,982
Accounts payable	950	(942)	604
Accrued expenses	105	2,788	537
Other non-current liabiliites		(189)	186
Lease incentive benefit			1,112
Net cash provided by (used in) operating activities	(2,725)	41,299	(11,918)
Cash flow from investing activities
Purchases of property and equipment		(1,030)	(2,988)
Change in restricted cash			(314)
Purchases of marketable securities		(220,399)
Proceeds from marketable securities		26,660
Net cash used in investing activities		(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	2,860	19,999	22,040
Proceeds from issuance of Series B redeemable convertible preferred stock, net of discount and issuance costs		84,780
Proceeds from issuance of common stock and restricted stock		10	80
Conversion of preferred stock to common stock		72,955
Net cash provided by financing activities	2,860	177,744	22,120
Net increase in cash and cash equivalents	135	24,274	6,900
Cash and cash equivalents, beginning of period		7,035	135
Cash and cash equivalents, end of period	$ 135	31,309	7,035
Supplemental Disclosure of Cash and non-cash Activities
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		$ 150,187	$ 2,929