Debt (Details) (USD $)	0 Months Ended			12 Months Ended			0 Months Ended		1 Months Ended	0 Months Ended
	Aug. 18, 2014	Jul. 22, 2013	Sep. 28, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 15, 2014	Jun. 29, 2012	Jun. 30, 2014	Dec. 27, 2012
Debt
Net proceeds from issuance of common stock (in shares)	10,235,000	6,900,000	6,304,102
Shares price (in dollars per share)	$ 11.25	$ 10.40	$ 4.07
Proceeds from issuance of common stock	$ 108,000,000	$ 67,000,000	$ 25,700,000	$ 109,013,000	$ 67,017,000	$ 25,658,000
Debt:
Notes payable				25,000,000
Accretion of end of term charge				314,000
Issuance fees paid to lender				(308,000)
Discount from warrant				(150,000)
Long-term debt				24,856,000	16,338,000
Future principal repayments of the Notes
2016				25,000,000
Total				25,000,000
Hercules
Debt
Net proceeds from issuance of common stock (in shares)							70,771
Shares price (in dollars per share)							$ 14.13
Proceeds from issuance of common stock							1,000,000
Loan and Security Agreement
Debt
Maximum borrowing capacity of term loan							25,000,000	20,000,000
Number of Secured Promissory Notes								2
Period over which principal payment could be delayed				5 months
Fee paid to extend the interest-only period									100,000
Commitment fee							25,000
Facility fee							125,000
Increase in borrowing capacity							5,000,000
Amount of cash proceeds required to extend maturity date of loan							90,000,000
Maturity date extension fee							250,000
"End of term" charge								390,000
Additional interest rate in event of default (as a percent)				5.00%
Limit on lender's participation rights in certain future private equity financings				1,000,000
Notes
Debt
Interest rate (as a percent)				9.25%
Note A
Debt
Incremental amount of term loan								10,000,000
Period over which note is to be repaid								42 months
Payment periods representing interest only								12 months
Periods of equal payment of principal and interest								30 months
Note B
Debt
Incremental amount of term loan										$ 10,000,000
Period over which note is to be repaid										36 months
Payment periods representing interest only										6 months
Periods of equal payment of principal and interest										30 months