Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net loss	$ (42,854)	$ (67,982)
Adjustments to reconcile net loss to cash used in operating activities:
Noncash interest expense	222	1,132
Noncash rent expense	(60)	(20)
Loss on extinguishment of debt		4,239
Amortization of premium on marketable securities	225	75
Depreciation	169	45
Change in fair value of warrant liabilities		(267)
Change in fair value of Convertible Bridge Notes redemption rights derivative		(480)
Change in fair value of 2020 Convertible Notes derivative liability		42,793
Stock-based compensation	2,909	2,380
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(948)	(1,256)
Accounts payable	(41)	487
Accrued expenses and other current liabilities	3,112	1,438
Net cash used in operating activities	(37,266)	(17,416)
Cash flows from investing activities:
Purchases of short-term investments	(122,277)	(33,693)
Proceeds from the maturities of short-term investments	56,705	2,430
Purchase of property and equipment	(487)	(412)
Net cash used in investing activities	(66,059)	(31,675)
Cash flows from financing activities:
Net proceeds from issuance of common stock in initial public offering		38,085
Proceeds from issuance of 2020 Convertible Notes in initial public offering		21,000
Net proceeds from issuance of common stock	3,997	73,965
Principal payments on notes payable		(5,800)
Net cash provided by financing activities:	53,081	125,515
Net change in cash and cash equivalents	(50,244)	76,424
Cash and cash equivalents, beginning of period	80,042	3,618
Cash and cash equivalents, end of period	29,798	80,042
Supplemental disclosure of cash flow information:
Cash paid for interest		89
Supplemental disclosure of noncash investing and financing activities:
Acquisition of leasehold improvements		445
Conversion of Notes into common stock		66,376
Net unrealized loss on marketable securities	43	11
2021 Convertible Notes [Member]
Cash flows from financing activities:
Proceeds from issuance of 2021 Convertible Notes	52,000
Payments of Notes issuance costs	(3,262)
2020 Convertible Notes [Member]
Adjustments to reconcile net loss to cash used in operating activities:
Loss on extinguishment of debt		3,716
Cash flows from financing activities:
Payments of Notes issuance costs		(1,841)
Stock Option Plans [Member]
Cash flows from financing activities:
Proceeds from the issuance of common stock	191	43
Employee Stock Purchase Plan [Member]
Cash flows from financing activities:
Proceeds from the issuance of common stock	$ 155	63
IPO
Supplemental disclosure of noncash investing and financing activities:
Conversion of Notes into common stock		2,027
Reclassification of fair value of warrant liability to equity		215
Reclassification of deferred public offering costs to stockholders’ equity		1,590
Reclassification of deferred public offering costs to other assets		256
Series AA Redeemable Convertible Preferred Stock [Member]
Supplemental disclosure of noncash investing and financing activities:
Accretion of Series AA preferred stock to redemption value		131
Series AA Redeemable Convertible Preferred Stock [Member] | IPO
Supplemental disclosure of noncash investing and financing activities:
Conversion of preferred stock into common stock upon initial public offering		46,384
Series X Redeemable Convertible Preferred Stock [Member] | IPO
Supplemental disclosure of noncash investing and financing activities:
Conversion of preferred stock into common stock upon initial public offering		$ 548