Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Cash flows from operating activities:
Net loss	$ (72,807)	$ (47,384)	$ (192,300)
Adjustments to reconcile net loss to net cash flows used in operating activities:
Stock-based compensation expense	6,994	6,229
Depreciation expense	460	104
Net accretion of discounts on marketable securities	(1,532)	(486)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	1,853	3,170
Operating lease assets	528	143
Accounts payable	(8,511)	2,629
Accrued expenses and other current liabilities	(6,800)	2,637
Operating lease liabilities	(555)	(148)
Other long-term liabilities	0	100
Net cash flows used in operating activities	(80,370)	(33,006)
Cash flows from investing activities:
Purchases of marketable securities	(203,527)	(179,607)
Maturities of marketable securities	203,120	0
Purchases of property and equipment	(669)	(41)
Increase in restricted investments	(614)	0
Net cash flows used in investing activities	(1,690)	(179,648)
Cash flows from financing activities:
Proceeds from public offerings, net of underwriting discounts and commissions	189,175	0
Repayment of notes payable to related party	0	(31)
Payments of public offering costs	(352)	0
Proceeds from exercise of stock options	2,569	1,149
Net cash flows provided by financing activities	191,392	1,118
Net increase (decrease) in cash and cash equivalents	109,332	(211,536)
Cash and cash equivalents at beginning of period	120,320	293,764	293,764
Cash and cash equivalents at end of period	229,652	82,228	$ 120,320
Supplemental disclosure of cash flow information:
Cash paid for interest	0	13
Property and equipment purchases included in accounts payable and accrued expenses and other current liabilities	152	0
Offering costs included in accounts payable and accrued expenses and other current liabilities	$ 438	$ 0