CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Activities:
Net loss	$ (132,694)	$ (184,748)	$ (183,769)
Reconciliation of net loss to net cash used in operating activities:
Depreciation and amortization	5,676	8,159	9,817
Loss (Gain) on disposal of property and equipment	88	(55)	269
Gain on Catalent transaction	(9,016)
Non-cash impact of lease termination		(4,381)
Amortization of debt issuance costs	1,424	1,424	1,424
Lease incentives received			1,933
Non-cash stock-based compensation	17,048	18,314	19,809
Other	4,869	(2,396)	2,715
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(4,202)	1,212	2,590
Accounts payable and accrued expenses	(11,485)	(6,744)	5,192
Deferred revenue	(8,331)	(15,610)	(4,456)
Net cash used in operating activities	(136,623)	(184,825)	(144,476)
Investing Activities:
Capital expenditures	(1,857)	(1,372)	(4,189)
Proceeds from Catalent transaction	18,333
Purchases of marketable securities	(17,484)	(120,150)	(218,045)
Proceeds from maturities of marketable securities	39,500	150,118	258,202
Net cash provided by investing activities	38,492	28,596	35,968
Financing Activities:
Principal payments of capital leases			(37)
Net proceeds from sales of common stock	97,392	100,060	63,425
Proceeds from the exercise of stock options and employee stock purchases	992	2,745	1,152
Net cash provided by financing activities	98,384	102,805	64,540
Effect of exchange rate on cash, cash equivalents and restricted cash	(32)	(48)	142
Net increase (decrease) in cash, cash equivalents and restricted cash	221	(53,472)	(43,826)
Cash, cash equivalents and restricted cash at beginning of year	81,959	135,431
Cash, cash equivalents and restricted cash at end of year	82,180	81,959	135,431
Supplemental disclosure of non-cash activities:
Sale of common stock under the Sales Agreement not settled at year-end	497
Capital expenditures included in accounts payable and accrued expenses	49	519	15
Supplemental disclosure of cash flow information:
Cash interest payments	$ 12,188	$ 12,188	$ 12,188