CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Operating Activities:
Net loss	$ (90,844)	$ (88,319)
Reconciliation of net loss to net cash used in operating activities:
Depreciation and amortization	4,136	4,167
(Gain) Loss on disposal of property and equipment	(45)	296
Non-cash impact of lease termination	(4,381)	0
Amortization of debt issuance costs	712	712
Lease incentives received	0	1,485
Non-cash stock-based compensation	9,496	8,726
Other	(1,067)	1,347
Changes in operating assets and liabilities:
Prepaid expenses and other assets	389	1,869
Accounts payable and accrued expenses	(6,504)	(105)
Deferred revenue	(17,938)	12,922
Net cash used in operating activities	(106,046)	(56,900)
Investing Activities:
Capital expenditures	(459)	(2,318)
Proceeds from maturities of marketable securities	51,115	130,609
Purchases of marketable securities	(78,374)	(151,044)
Net cash used in investing activities	(27,718)	(22,753)
Financing Activities:
Principal payments on capital lease	0	(37)
Net proceeds from sales of common stock	96,486	22,735
Proceeds from the exercise of stock options and employee stock purchases	1,442	797
Net cash provided by financing activities	97,928	23,495
Effect of exchange rate on cash, cash equivalents and restricted cash	(36)	61
Net decrease in cash, cash equivalents and restricted cash	(35,872)	(56,097)
Cash, cash equivalents and restricted cash at beginning of period	135,431	179,257
Cash, cash equivalents and restricted cash at end of period	99,559	123,160
Supplemental disclosure of non-cash activities:
Sale of common stock under the Sales Agreement not settled at quarter-end	0	334
Property and equipment purchases included in accounts payable and accrued expenses	161	686
Supplemental disclosure of cash flow information:
Cash payments of interest	$ 6,094	$ 6,094