Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities
Net loss	$ (511,585)	$ (214,494)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	122,552	70,323
Premium on marketable securities	(2,652)	(165)
Amortization of discount on marketable securities	(6,429)	(6,901)
Depreciation	1,618	780
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(8,494)	(12,271)
Other long-term assets	(3,818)
Right-of-use operating asset	6,424
Operating lease liabilities	(6,133)
Accounts payable	(28,955)	(3,370)
Accrued expenses and other liabilities	25,492	7,205
Net cash used in operating activities	(411,980)	(158,893)
Cash flows from investing activities
Proceeds from sales and maturities of marketable securities	957,840	724,132
Purchases of marketable securities	(1,077,702)	(1,272,959)
Purchases of property and equipment	(5,378)	(2,052)
Net cash used in investing activities	(125,240)	(550,879)
Cash flows from financing activities
Proceeds from stock option exercises and employee stock purchase plan issuances	46,684	27,480
Payment of employee tax obligations related to vesting of restricted stock units	(2,175)	(904)
Payments of offering costs	(328)	(340)
Proceeds from public offerings of common stock, net of commissions and underwriting discounts	561,277	631,494
Net cash provided by financing activities	605,458	657,730
Net increase (decrease) in cash, cash equivalents and restricted cash	68,238	(52,042)
Cash, cash equivalents and restricted cash at beginning of period	193,310	307,084
Cash, cash equivalents and restricted cash at end of period	261,548	255,042
Supplemental disclosure of non-cash operating and investing activities
Purchases of property and equipment included in accounts payable	11	$ 19
Right-of-use assets obtained in exchange for new operating lease liabilities	$ 872