Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended	48 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2015	Dec. 31, 2019	Oct. 27, 2014
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	$ 192,762	$ 194,532
Common stock options exercised	441
Reclassification out of accumulated other comprehensive income and non-controlling interest upon the deconsolidation of a subsidiary		(617)
Stock-based compensation	643	558
Net income (loss)	217	(1,023)
Other comprehensive income (loss)	(1,873)	2,531
Balance, end of period	192,190	195,981		$ 192,762
Reclassification adjustment from AOCI	0	0
Stock repurchase program, authorized amount					$ 5,000
Shares repurchased (in shares)			908,000	0
Average price of shares repurchased (in dollars per share)			$ 2.52
Total purchase price			$ 2,300
Stock repurchase program remaining authorized repurchase amount	2,700
Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	3,532	3,532
Balance, end of period	3,532	3,532		$ 3,532
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	41	40
Balance, end of period	41	40		41
Additional Paid-In Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	236,957	234,418
Common stock options exercised	441
Stock-based compensation	643	558
Balance, end of period	238,041	234,976		236,957
Accumulated Deficit
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(47,783)	(45,183)
Net income (loss)	(178)	(1,104)
Balance, end of period	(47,961)	(46,287)		(47,783)
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(4,862)	(1,972)
Reclassification out of accumulated other comprehensive income and non-controlling interest upon the deconsolidation of a subsidiary		(1,150)
Other comprehensive income (loss)	(1,775)	2,389
Balance, end of period	(6,637)	(733)		(4,862)
AXT, Inc. Stockholders' Equity
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	187,885	190,835
Common stock options exercised	441
Reclassification out of accumulated other comprehensive income and non-controlling interest upon the deconsolidation of a subsidiary		(1,150)
Stock-based compensation	643	558
Net income (loss)	(178)	(1,104)
Other comprehensive income (loss)	(1,775)	2,389
Balance, end of period	187,016	191,528		187,885
Noncontrolling Interests
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	4,877	3,697
Reclassification out of accumulated other comprehensive income and non-controlling interest upon the deconsolidation of a subsidiary		533
Net income (loss)	395	81
Other comprehensive income (loss)	(98)	142
Balance, end of period	$ 5,174	$ 4,453		$ 4,877