Redeemable Convertible Preferred Stock and Stockholders' Deficit (Details 2) (USD $)
 0 Months Ended 3 Months Ended 9 Months Ended 0 Months Ended
Aug. 07, 2013
Sep. 30, 2013
Dec. 31, 2012
Sep. 30, 2013
Redeemable preferred stock warrants
Other income (expense)
Sep. 30, 2013
Redeemable preferred stock warrants
Other income (expense)
Dec. 31, 2012
Common stock warrants
Transaction 1
Dec. 31, 2012
Common stock warrants
Transaction 2
Sep. 30, 2013
Common stock warrants
Transaction 3
Sep. 30, 2013
Common stock warrants
Transaction 4
Dec. 31, 2012
Preferred stock warrants
Series C redeemable convertible preferred stock
Dec. 31, 2012
Preferred stock warrants
Series E redeemable convertible preferred stock
Aug. 07, 2013
Preferred stock warrants
Series G-1 redeemable convertible preferred stock
Dec. 31, 2012
Preferred stock warrants
Series G-1 redeemable convertible preferred stock
Dec. 31, 2009
Preferred stock warrants
Series G-1 redeemable convertible preferred stock
Warrants to Purchase Stock and Preferred Stock Warrant Liability                            
Number of Shares Subject to Warrant   11,715 735,616     71,153 470,082 7,325 4,390 7,325 4,390   182,666 182,666
Exercise Price (in dollars per share)           $ 7.49 $ 9.93 $ 5.50 $ 11.39 $ 5.50 $ 11.39   $ 9.26 $ 9.26
Number of warrants exercised to purchase shares of common and preferred stock (in shares) 723,901                          
Fair value of warrant     $ 601,000                 $ 1,300,000   $ 400,000
Offering price (in dollars per share) $ 16.00                          
Change in fair value included in other income (expense)       $ (29,000) $ 700,000                  
Common stock, shares issued to warrant holders upon net exercise 293,232                     76,964