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	$ (163,458)	$ (89,195)	$ (43,847)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	41,271	27,528	17,903
Depreciation and amortization expense	2,995	1,263	447
Change in fair value of preferred stock warrant liability		2,187	785
Accretion of discount on investments	(2,320)	(329)
Loss on disposal of property and equipment	335
Non-cash interest expense	289	86	42
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	2,655	(9,037)	(647)
Operating lease, right-of-use-asset	4,991
Other assets	2	(74)
Accounts payable	(1,037)	4,938	1,110
Accrued expenses and other current liabilities	6,503	4,307	2,247
Deferred rent		(15)	22
Operating lease liabilities	(2,670)
Net cash used in operating activities	(110,444)	(58,341)	(21,938)
Cash flows from investing activities:
Purchases of property, plant and equipment	(40,657)	(14,952)	(2,251)
Purchases of investments	(319,133)	(160,972)
Sales and maturities of investments	227,155	64,285
Net cash used in investing activities	(132,635)	(111,639)	(2,251)
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs		100,986	120,067
Proceeds from initial public offering of common stock, net of commissions and underwriting discounts		257,866
Payments of initial public offering costs		(3,548)
Proceeds from the sale of restricted common stock			100
Proceeds from repayment of promissory note		246
Payments of debt issuance costs		(285)	(11)
Repurchase of unvested restricted common stock	(122)
Proceeds from borrowings under loan and security agreement	25,000	25,000	1,500
Payment of long-term debt		(5,500)
Proceeds from issuance of common stock upon exercise of stock options	2,413	64	37
Net cash provided by financing activities	27,291	374,829	121,693
Net increase (decrease) in cash, cash equivalents and restricted cash	(215,788)	204,849	97,504
Cash, cash equivalents and restricted cash at beginning of period	309,421	104,572	7,068
Cash, cash equivalents and restricted cash at end of period	93,633	309,421	104,572
Supplemental cash flow information:
Cash paid for interest, net of interest capitalized	2,961	385	265
Cash paid for leases	5,375
Supplemental disclosure of non-cash investing and financing information:
Lease assets obtained in exchange for new operating lease liabilities	49,799
Purchases of property, plant and equipment included in accounts payable or accrued expenses	3,095	1,550	9
Amounts capitalized under build-to-suit transaction		45,142
Proceeds from issuance of common stock upon exercise of stock options in other current assets	$ 32
Reclassification of warrant liability to additional paid-in capital		3,053
Conversion of preferred stock to common stock upon closing of the initial public offering		240,776
Issuance of preferred stock warrant in connection with loan and security agreement			14
Accretion of Series A redeemable convertible preferred stock to redemption value			$ 656
Debt issuance costs included in accounts payable and accrued expenses		$ 489