Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities
Net loss	$ (331,627)	$ (91,576)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	78,883	44,845
Premium on marketable securities	(1,887)	(75)
Amortization of discount on marketable securities	(4,973)	(4,154)
Depreciation	920	493
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(5,102)	(25,091)
Other long-term assets	(4,023)
Right of use operating asset	4,713
Operating lease liabilities	(4,346)
Accounts payable	(21,806)	(903)
Accrued expenses and other liabilities	5,970	(2,245)
Net cash used in operating activities	(283,278)	(78,706)
Cash flows from investing activities
Proceeds from sales and maturities of marketable securities	588,652	263,460
Purchases of marketable securities	(918,233)	(813,337)
Purchases of property and equipment	(3,438)	(1,054)
Net cash used in investing activities	(333,019)	(550,931)
Cash flows from financing activities
Proceeds from stock option exercises and employee stock purchase plan issuances	31,282	19,402
Payment of employee tax obligations related to vesting of restricted stock units	(692)	(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	591,539	649,652
Net increase (decrease) in cash, cash equivalents and restricted cash	(24,758)	20,015
Cash, cash equivalents and restricted cash at beginning of period	193,310	307,084
Cash, cash equivalents and restricted cash at end of period	168,552	327,099
Supplemental disclosure of non-cash operating and investing activities
Purchases of property and equipment included in accounts payable	252	$ 10
Right of use assets obtained in exchange for new operating lease liabilities	$ 872