Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Operating activities
Net loss	$ (16,741)	$ (20,986)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	67	32
Amortization of deferred financing costs and debt discount		93
Accretion of final interest payment on term loans		45
Stock-based compensation expense	4,176	2,840
Loss from disposal of property and equipment		2
Changes in operating assets and liabilities:
Accounts receivable	307	(2,021)
Prepaid expenses and other assets	(3,330)	(1,052)
Accounts payable	(1,367)	1,405
Accrued expenses and accrued final interest payment on term loan	427	(834)
Deferred revenue	469	760
Net cash used in operating activities	(15,992)	(19,716)
Investing activities
Purchases of property and equipment	(72)	(51)
Net cash used in investing activities	(72)	(51)
Financing activities
Proceeds from sale of common stock, net of issuance costs		162,445
Repayment of term loan payable		(4,646)
Proceeds from exercise of stock options	15	1,246
Net cash provided by financing activities	15	159,045
Net (decrease) increase in cash and cash equivalents	(16,049)	139,278
Cash and cash equivalents at beginning of period	205,912	121,042
Cash and cash equivalents at end of period	$ 189,863	260,320
Supplemental cash flow and noncash financing activities
Cash paid for interest		363
Issuance costs included in accounts payable and accrued expenses		$ 236