CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Operating Activities:
Net loss	$ (135,414)	$ (132,927)
Reconciliation of net loss to net cash used in operating activities:
Depreciation and amortization	6,177	7,696
(Gain) Loss on disposal of property and equipment	(55)	294
Non-cash impact of lease termination	(4,381)	0
Amortization of debt issuance costs	1,067	1,067
Lease incentives received	0	1,485
Non-cash stock-based compensation	13,927	13,057
Other	(1,820)	2,469
Changes in operating assets and liabilities:
Prepaid expenses and other assets	1,693	427
Accounts payable and accrued expenses	(10,497)	364
Deferred revenue	(10,256)	4,991
Net cash used in operating activities	(139,559)	(101,077)
Investing Activities:
Capital expenditures	(855)	(3,543)
Proceeds from maturities of marketable securities	98,305	189,817
Purchases of marketable securities	(117,172)	(167,069)
Net cash (used in) provided by investing activities	(19,722)	19,205
Financing Activities:
Principal payments on capital lease	0	(37)
Net proceeds from sales of common stock	96,486	46,029
Proceeds from the exercise of stock options and employee stock purchases	2,462	1,133
Net cash provided by financing activities	98,948	47,125
Effect of exchange rate on cash, cash equivalents and restricted cash	(62)	180
Net decrease in cash, cash equivalents and restricted cash	(60,395)	(34,567)
Cash, cash equivalents and restricted cash at beginning of period	135,431	179,257
Cash, cash equivalents and restricted cash at end of period	75,036	144,690
Supplemental disclosure of non-cash activities:
Sale of common stock under the Sales Agreement not settled at quarter-end	0	592
Property and equipment purchases included in accounts payable and accrued expenses	126	81
Supplemental disclosure of cash flow information:
Cash payments of interest	$ 12,188	$ 12,188