Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities
Net loss	$ (91,576)	$ (126,979)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	44,845	15,558
Premium on marketable securities	(75)
Amortization of premium (discount) on marketable securities	(4,154)	5
Depreciation	493	260
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(25,091)	(93)
Accounts payable	(903)	(6,768)
Accrued expenses and other liabilities	(2,245)	3,731
Net cash used in operating activities	(78,706)	(114,286)
Cash flows from investing activities
Proceeds from sales and maturities of marketable securities	263,460	110,436
Purchases of marketable securities	(813,337)	(33,922)
Purchases of property and equipment	(1,054)	(321)
Net cash provided by (used in) investing activities	(550,931)	76,193
Cash flows from financing activities
Proceeds from stock option exercises and employee stock purchase plan issuances	19,402	3,311
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	649,652	3,311
Net increase (decrease) in cash, cash equivalents and restricted cash	20,015	(34,782)
Cash, cash equivalents and restricted cash at beginning of period	307,084	169,081
Cash, cash equivalents and restricted cash at end of period	327,099	$ 134,299
Supplemental disclosure of non-cash investing and financing activities
Purchases of property and equipment included in accounts payable	$ 10