Stockholders' Equity (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 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	66,000
Stock-based compensation	467,000
Net income	3,190,000	$ 425,000
Other comprehensive income	2,699,000	352,000
Balance, end of period	194,739,000		$ 188,317,000
Reclassification adjustment from AOCI	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
Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	3,532,000
Common stock options exercised
Balance, end of period	3,532,000		$ 3,532,000
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	39,000
Common stock options exercised
Balance, end of period	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	66,000
Stock-based compensation	467,000
Balance, end of period	232,212,000		231,679,000
Accumulated Deficit
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(54,837,000)
Net income	2,875,000
Balance, end of period	(51,962,000)		(54,837,000)
Accumulated Other Comprehensive Income
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	3,407,000
Other comprehensive income	2,439,000
Balance, end of period	5,846,000		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	66,000
Stock-based compensation	467,000
Net income	2,875,000
Other comprehensive income	2,439,000
Balance, end of period	189,667,000		183,820,000
Noncontrolling Interests
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	4,497,000
Net income	315,000
Other comprehensive income	260,000
Balance, end of period	$ 5,072,000		$ 4,497,000