Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Cash flows from operating activities
Net loss	$ (25,349)	$ (15,073)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities
Depreciation and amortization	768	621
Amortization of premium on marketable securities	642
Stock-based compensation	3,386	1,110
Abandonment of capitalized intangible assets	280	496
Gain on disposal of assets	(9)	(1)
Gain on sale of marketable securities available-for-sale	(4)
Changes in operating assets and liabilities:
Accounts receivable	2,558	40
Interest receivable	(659)
Prepaid expenses and other assets	(715)	(62)
Accounts payable	3,417	(536)
Accrued expenses	1,608	372
Deferred rent	595
Deferred revenue	43,993	(2,454)
Net cash provided by (used in) operating activities	30,511	(15,487)
Cash flows from investing activities
Purchase of marketable securities	(168,489)
Purchase of intangible assets	(1,220)	(1,137)
Purchase of property and equipment	(1,724)	(560)
Proceeds from sale of marketable securities available-for-sale	28,008
Proceeds from sale of property and equipment	9	1
Net cash used in investing activities	(143,416)	(1,696)
Cash flows from financing activities
Proceeds from issuance of common stock	122,906
Proceeds from issuance of common stock upon exercise of stock awards	543	8
Proceeds from issuance of common stock under the Employee Stock Purchase Plan	247	130
Common stock issuance cost	(7,702)
Payments on capital lease obligations		(7)
Net cash provided by financing activities	115,994	131
Net increase (decrease) in cash and cash equivalents	3,089	(17,052)
Cash and cash equivalents, beginning of period	54,649	77,975
Cash and cash equivalents, end of period	57,738	$ 60,923
Supplemental disclosures of non-cash investing activities
Net unrealized loss on marketable securities available for sale	$ 6