CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities:
Net loss	$ 48,486	$ 32,581
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	8,488	9,478
Depreciation and amortization expense	1,789	661
Accretion of discount on investments	(29)	(489)
Non-cash interest expense	87	21
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	914	2,695
Operating lease, right-of-use-asset	1,579	509
Accounts payable	905	(966)
Accrued expenses and other current liabilities	(3,954)	(2,487)
Operating lease liabilities	(1,308)	287
Net cash used in operating activities	(40,015)	(22,872)
Cash flows from investing activities:
Purchases of property, plant and equipment	(2,851)	(4,164)
Purchases of investments	(10,001)	(117,983)
Sales and maturities of investments	105,000	29,155
Net cash provided by (used in) investing activities	92,148	(92,992)
Cash flows from financing activities:
Proceeds from issuance of common stock upon exercise of stock options	486	799
Net cash provided by financing activities	486	799
Net increase (decrease) in cash, cash equivalents and restricted cash	52,619	(115,065)
Cash, cash equivalents and restricted cash at beginning of period	93,633	309,421
Cash, cash equivalents and restricted cash at end of period	146,252	194,356
Supplemental cash flow information:
Cash paid for interest, net of interest capitalized	895	336
Cash paid for leases	2,147	177
Supplemental disclosure of non-cash investing and financing information:
Lease assets obtained in exchange for new operating lease liabilities		27,944
Lease asset derecognized upon lease cancellation	982
Purchases of property, plant and equipment included in accounts payable or accrued expenses	$ 923	$ 4,630