Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Activities
Net income (loss)	$ 63,087	$ 114,205	$ (193,420)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities
Depreciation	183	2,763	757
Amortization of premium and discounts on investment securities	0	140	595
Non-cash charge on acquired in-process research and development	5,619	0	0
Non-cash gain on extinguishment of royalty purchase agreement	(125,000)	0	0
Deferred income taxes	0	(3,366)	22,954
Share-based compensation	11,072	18,171	31,660
Changes in operating assets and liabilities:
Income tax receivable	1,387	(1,387)	3,421
Due from Novartis Pharma AG	0	3,531	858
Prepaid expense and other current assets	1,060	(68)	(995)
Accrued interest receivable	0	466	203
Other assets	10	438	27
Accrued research and development expenses	2,353	(42,256)	28,420
Accounts payable and accrued expenses	(1,682)	(4,481)	14
Deferred revenue	0	(209,977)	(3,090)
Net cash used in operating activities	(41,911)	(121,821)	(108,596)
Investing Activities
Purchase of marketable securities	0	(12,014)	(72,197)
Maturities of marketable securities	0	166,806	86,500
Purchase of property and equipment	0	0	(572)
Net cash provided by investing activities	0	154,792	13,731
Financing Activities
Proceeds from issuance of common stock related to acquisition	6,075	0	0
Proceeds from employee stock plan purchases and stock option exercises	65	71	6,934
Net cash provided by financing activities	6,140	71	6,934
Net change in cash and cash equivalents	(35,771)	33,042	(87,931)
Cash and cash equivalents
Beginning of period	166,972	133,930	221,861
End of period	131,201	166,972	133,930
Supplemental disclosure of cash paid
Income taxes received, net	(2,467)	(245)	(26,998)
Supplemental disclosures of non-cash information related to investing activities
Change in unrealized gain (loss) on available for sale securities, net of tax	0	212	261
Common stock issued to acquire net assets of Inception 4, Inc.	$ 11,694	$ 0	$ 0