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	$ (680,238)	$ (372,882)	$ (270,120)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	153,231	101,963	35,534
Premium on marketable securities	(3,674)	(215)	(105)
Amortization of discount on marketable securities	(6,966)	(9,892)	(293)
Depreciation	2,283	1,143	531
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(4,781)	(15,462)	(1,127)
Other long-term assets	(3,793)
Right-of-use operating asset	8,168
Operating lease liabilities, current	2,804
Operating lease liabilities, non-current	(10,761)
Accounts payable	(18,783)	24,544	(3,693)
Accrued expenses and other liabilities	33,804	10,130	19,996
Net cash used in operating activities	(528,706)	(260,671)	(219,277)
Cash flows from investing activities
Proceeds from sales and maturities of marketable securities	1,171,270	974,757	261,225
Purchases of marketable securities	(1,308,675)	(1,484,358)	(244,403)
Purchases of property and equipment	(5,751)	(2,860)	(1,360)
Net cash provided by (used in) investing activities	(143,156)	(512,461)	15,462
Cash flows from financing activities
Proceeds from stock option exercises and employee stock purchase plan issuances	48,850	29,108	15,972
Payment of employee tax obligations related to vesting of restricted stock units	(2,175)	(904)
Payments of offering costs	(328)	(340)	(179)
Proceeds from public offerings of common stock, net of commissions and underwriting discounts	561,277	631,494	326,025
Net cash provided by financing activities	607,624	659,358	341,818
Net increase (decrease) in cash, cash equivalents and restricted cash	(64,238)	(113,774)	138,003
Cash, cash equivalents and restricted cash at beginning of period	193,310	307,084	169,081
Cash, cash equivalents and restricted cash at end of period	129,072	193,310	307,084
Supplemental disclosure of non-cash operating and investing activities
Purchases of property and equipment included in accounts payable	65	51	138
Right-of-use assets obtained in exchange for new operating lease liabilities	$ 872
Landlord tenant incentive included in other current assets		$ 229
Purchases of property and equipment financed with landlord tenant incentive			1,665
Public offering costs included in accounts payable			$ 95