Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Activities
Net loss	$ (346,028)	$ (314,670)	$ (198,471)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:
Depreciation	7,172	6,432	5,708
Stock-based compensation expense	73,357	47,809	42,086
Net (accretion of discount) amortization of premium on marketable securities	(3,837)	(11)	773
Loss on disposal of property and equipment	20	40	0
Changes in operating assets and liabilities:
Accounts receivable, net	(5,076)	0	0
Collaboration receivable – related party	(14)	2,438	3,339
Collaboration receivable – other	(670)	0	0
Royalty receivable – related party	(1,012)	(1,222)	0
Inventory	(869)	0	0
Prepaid expenses and other current and non-current assets	1,148	(3,600)	(2,420)
Accounts payable	(5,488)	5,329	3,501
Accrued expenses	8,623	1,522	16,854
Deferred revenue – related party	(31,665)	(26,570)	165,846
Deferred rent	(82)	(2,729)	1,346
Net cash (used in) provided by operating activities	(304,421)	(285,232)	38,562
Investing Activities
Purchases of marketable securities	(933,320)	(688,702)	(506,067)
Proceeds from maturities and sales of marketable securities	666,481	635,421	396,632
Purchases of property and equipment	(6,986)	(4,627)	(9,915)
Net cash used in investing activities	(273,825)	(57,908)	(119,350)
Financing Activities
Proceeds from public offering of common stock, net of reimbursements	516,206	270,250	162,150
(Payment) of public offering costs	(391)		(230)
Reiumbursement of public offering costs		638
Net proceeds from stock option exercises and employee stock purchase plan	30,209	14,222	7,858
Net cash provided by financing activities	546,024	285,110	169,778
Net change in cash and cash equivalents	(32,222)	(58,030)	88,990
Cash and cash equivalents at beginning of the period	102,724	160,754	71,764
Cash and cash equivalents at end of the period	70,502	102,724	160,754
Supplemental disclosure of non-cash investing and financing transactions:
Additions to property and equipment in accounts payable and accrued expenses	1,106	1,011	73
Proceeds from stock option exercises in other current assets	7	0	32
Public offering costs in other receivables, net of amounts in accounts payable and accrued expenses	$ 0	$ 0	$ 230