CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Cash flows from operating activities:
Net loss		$ (118,986)	$ (62,012)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense		30,440	18,402
Depreciation and amortization expense		2,294	879
Change in fair value of preferred stock warrant liability	$ 426		2,187
Accretion of discount on investments		(1,939)	(198)
Loss on disposal of property and equipment		119
Non-cash interest expense		201	26
Changes in operating assets and liabilities:
Prepaid expenses and other current assets		(2,129)	(2,792)
Operating lease, right-of-use-asset		3,336
Other assets		(293)
Accounts payable		3,184	2,402
Accrued expenses and other current liabilities		3,725	2,472
Deferred rent			(14)
Operating lease liabilities		(1,411)
Net cash used in operating activities		(81,459)	(38,648)
Cash flows from investing activities:
Purchases of property, plant and equipment		(27,381)	(11,671)
Purchases of investments		(259,231)	(87,369)
Sales and maturities of investments		143,275	33,460
Net cash used in investing activities		(143,337)	(65,580)
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs			100,986
Proceeds from initial public offering of common stock, net of commissions and underwriting discounts			257,866
Payments of initial public offering costs			(3,449)
Proceeds from repayment of promissory note			245
Repurchase of unvested restricted common stock		(122)
Proceeds from borrowings under loan and security agreement		25,000
Proceeds from issuance of common stock upon exercise of stock options		1,843	18
Net cash provided by financing activities		26,721	355,666
Net increase (decrease) in cash, cash equivalents and restricted cash		(198,075)	251,438
Cash, cash equivalents and restricted cash at beginning of period		309,421	104,572	$ 104,572
Cash, cash equivalents and restricted cash at end of period	$ 356,010	111,346	356,010	$ 309,421
Supplemental cash flow information:
Cash paid for interest, net of interest capitalized		1,706
Cash paid for leases		3,194
Supplemental disclosure of non-cash investing and financing information:
Lease assets obtained in exchange for new operating lease liabilities		49,496
Purchases of property, plant and equipment included in accounts payable or accrued expenses		6,073	1,353
Amounts capitalized under build-to-suit lease transaction			24,657
Deferred offering costs and issuance costs included in accounts payable or accrued expenses		$ 11
Offering costs and issuance costs included in accounts payable and accrued expenses			99
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