CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Operating activities
Net loss	$ (26,441)	$ (16,863)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	290	359
Stock-based compensation expense	2,410	3,433
Amortization of deferred financing costs, debt discounts and premiums and discounts on available-for-sale marketable securities	105	(92)
Changes in operating assets and liabilities:
Prepaid expenses, other current assets and other assets	(1,479)	(41)
Accounts payable	2,730	(1,832)
Accrued expenses and other liabilities	(2,748)	(2,408)
Liability classified stock-based compensation awards		(69)
Net cash used in operating activities	(25,133)	(17,513)
Investing activities
Purchase of investments	(6,461)	(40,398)
Maturities of investments	24,580	72,610
Net cash provided by investing activities	18,119	32,212
Financing activities
Proceeds from long-term debt, net	2,386
Deferred debt financing costs	(138)
Cash used to settle restricted stock liability		(5)
Net cash provided by (used in) financing activities	2,248	(5)
(Decrease) increase in cash and cash equivalents	(4,766)	14,694
Cash and cash equivalents at beginning of period	32,349	24,870
Cash and cash equivalents at end of period	$ 27,583	39,564
Supplemental disclosure of non-cash investing and financing activities
Purchase of property and equipment in accounts payable		$ 39