Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities
Net loss	$ (218,051)	$ (83,012)	$ (38,474)
Adjustments to reconcile net loss to net cash used in operating activities:
Loss on common stock settlement	11,647
Depreciation and amortization	2,270	64	14
Stock-based compensation	31,544	26,786	5,250
Stock-based third-party research and development			20,154
Disposal of fixed assets	629	24
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(3,667)	(3,854)	(887)
Other long-term assets	(1,771)	(1,132)	(1,312)
Accounts payable	6,374	958	276
Accrued expenses	15,308	11,773	1,183
Accrued indemnification obligation	(1,664)	372
Net cash used in operating activities, Total	(157,381)	(48,021)	(13,796)
Cash flows from investing activities
Capital expenditures	(32,047)	(19,182)	(221)
Net cash used in investing activities, Total	(32,047)	(19,182)	(221)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Proceeds from exercise of stock options	3,689	430	520
Shares surrendered for taxes	(443)
Proceeds from exercise of stock warrants		53	341
Payments of deferred IPO costs		(1,592)
Proceeds from IPO and Follow-On Offering, net of issuance costs	269,869	246,490
Net cash provided by financing activities, Total	273,115	245,381	73,149
Net (decrease) increase in cash and cash equivalents	83,687	178,178	59,132
Cash and cash equivalents, beginning of period	240,430	62,252	3,120
Cash and cash equivalents, end of period	324,117	240,430	62,252
Supplemental cash flow information:
Capital expenditures in accounts payable and accrued liabilities	7,697	$ 4,872
Deferred issuance costs for underwritten offering in accounts payable and accrued liabilities	$ 83		371
Cash paid for interest			15
Class B Preferred Stock
Net Cash Provided by (Used in) Financing Activities [Abstract]
Proceeds from issuance of common stock, net of issuance costs			2,500
Class D Preferred Stock
Net Cash Provided by (Used in) Financing Activities [Abstract]
Proceeds from issuance of common stock, net of issuance costs			64,788
Class C Preferred Stock
Net Cash Provided by (Used in) Financing Activities [Abstract]
Proceeds from issuance of common stock, net of issuance costs			$ 5,000