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Description of Operations and Summary of Significant Accounting Policies - Additional Information (Detail)
$ / shares in Units, $ in Millions
 3 Months Ended 9 Months Ended
Nov. 19, 2014
USD ($)
$ / shares
shares
Nov. 10, 2014
Sep. 30, 2015
$ / shares
shares
Sep. 30, 2014
shares
Sep. 30, 2015
Segment
$ / shares
shares
Sep. 30, 2014
shares
Dec. 31, 2014
$ / shares
shares
Accounting Policy [Line Items]              
Common stock shares sold 9,315,000            
Public offering price | $ / shares $ 18.00            
Net proceeds from initial public offering and concurrent private placement | $ $ 171.8            
Underwriting discounts and commissions | $ 11.7            
Offering expenses | $ 4.1            
Common stock, aggregate purchase price | $ $ 20.0            
Convertible preferred stock converted into shares of common stock 33,919,954            
Number of operating segment | Segment         1    
Preferred stock, shares authorized     125,000,000   125,000,000   125,000,000
Preferred stock, par value | $ / shares     $ 0.01   $ 0.01   $ 0.01
Preferred stock, shares outstanding     0   0   0
Anti-dilutive securities excluded from computation of diluted net loss per share     15,000,000 48,000,000 15,000,000 48,000,000  
FibroGen Europe [Member]              
Accounting Policy [Line Items]              
Convertible preferred stock converted into shares of common stock 958,996            
Common stock [Member]              
Accounting Policy [Line Items]              
Description reverse stock split         On November 10, 2014, we effected a 1-for-2.5 reverse split of our common stock. Upon the effectiveness of the reverse stock split, (i) every 2.5 shares of outstanding common stock were combined into one share of common stock, (ii) the number of shares of common stock for which each outstanding option or warrant to purchase common stock is exercisable was proportionally decreased on a 1-for-2.5 basis, (iii) the exercise price of each outstanding option or warrant to purchase common stock was proportionately increased on a 1-for- 2.5 basis, (iv) the exchange ratio for each share of outstanding FibroGen Europe Oy (“FibroGen Europe”) share of stock which is exchangeable into our common stock was proportionately reduced on a 1-for-2.5 basis, and (v) the conversion ratio for each share of outstanding preferred stock which is convertible into our common stock was proportionately reduced on a 1-for-2.5 basis. All of the outstanding common stock share numbers (including shares of common stock which our outstanding preferred stock shares were convertible into), common stock warrants, share prices, exercise prices and per share amounts have been adjusted in these condensed consolidated financial statements, on a retroactive basis, to reflect this 1-for-2.5 reverse stock split for all periods presented. The par value per share and the authorized number of shares of common stock and preferred stock were not adjusted as a result of the reverse stock split.    
Reverse stock split ratio   0.4