CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Operating Activities:
Net loss	$ (82,821)	$ (90,844)
Reconciliation of net loss to net cash used in operating activities:
Depreciation and amortization	3,864	4,136
Loss (Gain) on disposal of property and equipment	88	(45)
Non-cash impact of lease termination		(4,381)
Amortization of debt issuance costs	712	712
Non-cash stock-based compensation	10,179	9,497
Other	1,269	(1,067)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	2,617	389
Accounts payable and accrued expenses	(10,952)	(6,505)
Deferred revenue	(5,577)	(17,938)
Net cash used in operating activities	(80,621)	(106,046)
Investing Activities:
Capital expenditures	(1,281)	(459)
Proceeds from maturities of marketable securities	24,500	51,115
Purchases of marketable securities	(2,484)	(78,374)
Net cash provided by (used in) investing activities	20,735	(27,718)
Financing Activities:
Net proceeds from sales of common stock	55,242	96,486
Proceeds from the exercise of stock options and employee stock purchases	942	1,442
Net cash provided by financing activities	56,184	97,928
Effect of exchange rate on cash, cash equivalents and restricted cash	(22)	(36)
Net decrease in cash, cash equivalents and restricted cash	(3,724)	(35,872)
Cash, cash equivalents and restricted cash at beginning of period	81,959	135,431
Cash, cash equivalents and restricted cash at end of period	78,235	99,559
Supplemental disclosure of non-cash activities:
Property and equipment purchases included in accounts payable and accrued expenses	146	161
Supplemental disclosure of cash flow information:
Cash payments of interest	$ 6,094	$ 6,094