Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities
Net loss	$ (214,494)	$ (200,698)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	70,323	25,289
Premium on marketable securities	(165)	(49)
Amortization of discount on marketable securities	(6,901)	(57)
Depreciation	780	396
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(12,271)	208
Accounts payable	(3,370)	221
Accrued expenses and other liabilities	7,205	14,312
Net cash used in operating activities	(158,893)	(160,378)
Cash flows from investing activities
Proceeds from sales and maturities of marketable securities	724,132	214,436
Purchases of marketable securities	(1,272,959)	(93,816)
Purchases of property and equipment	(2,052)	(556)
Net cash provided by (used in) investing activities	(550,879)	120,064
Cash flows from financing activities
Proceeds from stock option exercises and employee stock purchase plan issuances	27,480	6,998
Payment of employee tax obligations related to vesting of restricted stock units	(904)
Payments of offering costs	(340)
Proceeds from public offerings of common stock, net of commissions and underwriting discounts	631,494
Net cash provided by financing activities	657,730	6,998
Net decrease in cash, cash equivalents and restricted cash	(52,042)	(33,316)
Cash, cash equivalents and restricted cash at beginning of period	307,084	169,081
Cash, cash equivalents and restricted cash at end of period	255,042	135,765
Supplemental disclosure of non-cash investing activities
Purchases of property and equipment financed with landlord tenant incentive		$ 353
Purchases of property and equipment included in accounts payable	$ 19