Condensed Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash flows from operating activities:
Net loss	$ (49,520)	$ (29,044)	$ (13,510)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	450	231	93
Impairment of property and equipment		364
Amortization of premium on debt securities	871	586	77
Stock-based compensation expense	12,407	5,094	1,382
Issuance of common stock for technology access fee		2,500
Net loss on equity method investment	1,583
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(997)	(76)	(530)
Accounts payable	2,317	(721)	599
Accrued expenses and other non-current liabilities	2,301	2,972	733
Net cash used in operating activities	(30,588)	(18,094)	(11,156)
Cash flows from investing activities:
Investment in joint venture	(1,500)
Purchases of property and equipment	(2,804)	(719)	(849)
Maturities of short-term investments	20,797	5,670
Sales of short-term investments	8,431
Purchases of short-term investments	(57,603)	(15,974)	(16,698)
(Increase) decrease in restricted cash	(109)	5	(93)
Net cash used in investing activities	(32,788)	(11,018)	(17,640)
Cash flows from financing activities:
Proceeds from issuance of preferred stock, net of issuance costs			34,992
Net proceeds from the issuance of common stock	51,733	32,414	4,039
Issuances of common stock under benefit plans, net of withholding taxes paid	(21)
Net cash provided by financing activities	51,712	32,414	39,031
Net (decrease) increase in cash and cash equivalents	(11,664)	3,302	10,235
Cash and cash equivalents at beginning of period	18,078	14,776	4,541
Cash and cash equivalents at end of period	6,414	18,078	14,776
Supplemental disclosure of non-cash investing and financing activity
Conversion of convertible preferred stock to common stock			41,192
Investment in OvaXon		$ 1,500
Vesting of restricted stock, net of shares withheld for taxes	939000