CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating Activities:
Net loss	$ (46,352)	$ (43,854)
Reconciliation of net loss to net cash used in operating activities:
Depreciation and amortization	2,079	2,111
(Gain) Loss on disposal of property and equipment	(47)	65
Non-cash impact of lease termination	(4,381)	0
Amortization of debt issuance costs	356	356
Lease incentives received	0	1,248
Non-cash stock-based compensation	5,245	4,213
Other	(435)	(6)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	0	1,093
Accounts payable and accrued expenses	(13,293)	(4,867)
Deferred revenue	(9,247)	(5,399)
Other liabilities	0	543
Net cash used in operating activities	(66,075)	(44,497)
Investing Activities:
Capital expenditures	(150)	(1,084)
Proceeds from maturities of marketable securities	22,721	86,130
Purchases of marketable securities	(1,984)	(110,330)
Net cash provided by (used in) investing activities	20,587	(25,284)
Financing Activities:
Principal payments on capital lease	0	(18)
Net proceeds from sales of common stock	42,599	14,585
Proceeds from the exercise of stock options and employee stock purchases	1,325	797
Net cash provided by financing activities	43,924	15,364
Effect of exchange rate on cash, cash equivalents and restricted cash	(26)	31
Net decrease in cash, cash equivalents and restricted cash	(1,590)	(54,386)
Cash, cash equivalents and restricted cash at beginning of period	135,431	179,257
Cash, cash equivalents and restricted cash at end of period	133,841	124,871
Supplemental disclosure of non-cash activities:
Sale of common stock under the Sales Agreement not settled at quarter-end	0	225
Property and equipment purchases included in accounts payable and accrued expenses	62	823
Supplemental disclosure of cash flow information:
Cash payments of interest	$ 6,094	$ 6,094