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	$ (126,740)	$ (163,406)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	31,110	44,116
Premium on marketable securities	(240)	(739)
Amortization of discount on marketable securities	(18)	(2,322)
Depreciation	669	415
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	1,221	1,637
Other long-term assets	99	(1,378)
Right-of-use operating asset	1,778	2,942
Operating lease liabilities, current	25	(156)
Operating lease liabilities, non-current	(1,897)	(2,344)
Accounts payable	(7,307)	(20,507)
Accrued expenses and other liabilities	(35,391)	(8,192)
Net cash used in operating activities	(136,691)	(149,934)
Cash flows from investing activities
Proceeds from sales and maturities of marketable securities	275,197	307,263
Purchases of marketable securities	(68,651)	(580,916)
Purchases of property and equipment	(276)	(1,257)
Net cash provided by (used in) investing activities	206,270	(274,910)
Cash flows from financing activities
Proceeds from stock option exercises and employee stock purchase plan issuances	3,160	15,681
Payment of employee tax obligations related to vesting of restricted stock units		(692)
Payments of offering costs		(70)
Proceeds from public offerings of common stock, net of commissions and underwriting discounts		561,277
Net cash provided by financing activities	3,160	576,196
Net increase in cash, cash equivalents and restricted cash	72,739	151,352
Cash, cash equivalents and restricted cash at beginning of period	129,072	193,310
Cash, cash equivalents and restricted cash at end of period	201,811	344,662
Supplemental disclosure of non-cash operating and investing activities
Purchases of property and equipment included in accounts payable	$ 15	78
Public offering costs included in accounts payable		$ 259