Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Cash flows from operating activities
Net loss	$ (234,428)	$ (187,088)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	3,239	2,392
Noncash lease expense	2,823	2,337
Stock-based compensation	36,387	23,961
Accretion of premiums and discounts on investments	(26)	(2,973)
Other	269
Changes in assets and liabilities:
Accounts receivable	(2,740)	(686)
Unbilled accounts receivable	1,872	(4,363)
Inventory	(2,748)
Prepaid expenses and other current assets	(9,159)	(4,840)
Other assets	2,018	(179)
Accounts payable	802	3,291
Accrued expenses	(4,573)	9,994
Deferred revenue	(1,368)	(1,697)
Operating lease liabilities	(2,860)	(1,565)
Net cash used in operating activities	(210,492)	(161,416)
Cash flows from investing activities
Purchases of property and equipment	(2,466)	(3,940)
Purchases of investments	(232,148)	(465,289)
Maturities of investments	306,035	310,490
Net cash provided by (used in) investing activities	71,421	(158,739)
Cash flows from financing activities
Proceeds from public offerings of common stock, net of issuance cost	308,750	327,750
Net proceeds from stock option exercises and employee stock purchase plan	4,886	6,911
Payment of offering costs	(311)	(280)
Other financing activities		(72)
Net cash provided by financing activities	313,325	334,309
Net increase in cash, cash equivalents, and restricted cash	174,254	14,154
Cash, cash equivalents and restricted cash at beginning of period	119,604	73,429
Effect of exchange rate changes on cash, cash equivalents and restricted cash	(12)	3
Cash, cash equivalents and restricted cash at end of period	293,846	87,586
Supplemental cash flow information
Public offering costs incurred but unpaid at period end	5	28
Property and equipment purchases unpaid at period end	116	3,328
Cash paid for taxes, net	$ 47	$ 97