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Stockholders' Equity (Details) - USD ($)
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Oct. 27, 2014
Increase (Decrease) in Stockholders' Equity [Roll Forward]                
Balance, beginning of period     $ 188,317,000          
Common stock options exercised     264,000          
Purchase of subsidiary shares from noncontrolling interest     (1,607,000)          
Stock-based compensation     1,392,000          
Net income $ 4,147,000 $ 4,565,000 11,888,000 $ 6,788,000        
Other comprehensive loss (3,257,000) 1,162,000 (5,536,000) 2,359,000        
Balance, end of period 194,718,000   194,718,000   $ 188,317,000      
Reclassification adjustment from AOCI 0 $ 0 0 $ 0        
Stock repurchase program, authorized amount               $ 5,000,000
Shares repurchased (in shares)         0 0 908,000  
Average price of shares repurchased (in dollars per share)             $ 2.52  
Total purchase price             $ 2,300,000  
Stock repurchase program remaining authorized repurchase amount 2,700,000   2,700,000          
Preferred Stock                
Increase (Decrease) in Stockholders' Equity [Roll Forward]                
Balance, beginning of period     3,532,000          
Balance, end of period 3,532,000   3,532,000   $ 3,532,000      
Common Stock                
Increase (Decrease) in Stockholders' Equity [Roll Forward]                
Balance, beginning of period     39,000          
Balance, end of period 39,000   39,000   39,000      
Additional Paid-In Capital                
Increase (Decrease) in Stockholders' Equity [Roll Forward]                
Balance, beginning of period     231,679,000          
Common stock options exercised     264,000          
Purchase of subsidiary shares from noncontrolling interest     187,000          
Stock-based compensation     1,392,000          
Balance, end of period 233,522,000   233,522,000   231,679,000      
Accumulated Deficit                
Increase (Decrease) in Stockholders' Equity [Roll Forward]                
Balance, beginning of period     (54,837,000)          
Net income     10,715,000          
Balance, end of period (44,122,000)   (44,122,000)   (54,837,000)      
Accumulated Other Comprehensive Income (Loss)                
Increase (Decrease) in Stockholders' Equity [Roll Forward]                
Balance, beginning of period     3,407,000          
Other comprehensive loss     (5,253,000)          
Balance, end of period (1,846,000)   (1,846,000)   3,407,000      
AXT, Inc. Stockholders' Equity                
Increase (Decrease) in Stockholders' Equity [Roll Forward]                
Balance, beginning of period     183,820,000          
Common stock options exercised     264,000          
Purchase of subsidiary shares from noncontrolling interest     187,000          
Stock-based compensation     1,392,000          
Net income     10,715,000          
Other comprehensive loss     (5,253,000)          
Balance, end of period 191,125,000   191,125,000   183,820,000      
Noncontrolling Interests                
Increase (Decrease) in Stockholders' Equity [Roll Forward]                
Balance, beginning of period     4,497,000          
Purchase of subsidiary shares from noncontrolling interest     (1,794,000)          
Net income     1,173,000          
Other comprehensive loss     (283,000)          
Balance, end of period $ 3,593,000   $ 3,593,000   $ 4,497,000