Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended		12 Months Ended	48 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2015	Dec. 31, 2019	Oct. 27, 2014
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	$ 194,054	$ 192,190	$ 192,762	$ 195,655	$ 195,981	$ 194,532	$ 192,762	$ 194,532
Common stock options exercised	624	72	441	35	232
Purchase of subsidiary shares from noncontrolling interest					(413)
Sale of subsidiary shares to noncontrolling interests	396
Reclassification out of accumulated other comprehensive income and non-controlling interest upon the deconsolidation of a subsidiary						(617)		(617)
Stock-based compensation	648	641	643	583	568	558
Net income (loss)	1,481	959	217	(495)	1,738	(1,023)	2,657	220
Net dividend declared by joint ventures		(89)
Other comprehensive income (loss)	4,689	281	(1,873)	(4,324)	(2,451)	2,531
Balance, end of period	201,892	194,054	192,190	191,454	195,655	195,981	201,892	191,454		$ 192,762
Reclassification adjustment from AOCI	0			0			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						2,700
Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	3,532	3,532	3,532	3,532	3,532	3,532	3,532	3,532
Balance, end of period	3,532	3,532	3,532	3,532	3,532	3,532	3,532	3,532		$ 3,532
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	41	41	41	40	40	40	41	40
Balance, end of period	41	41	41	40	40	40	41	40		41
Additional Paid-In Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	238,754	238,041	236,957	235,702	234,976	234,418	236,957	234,418
Common stock options exercised	624	72	441	35	232
Purchase of subsidiary shares from noncontrolling interest					(74)
Sale of subsidiary shares to noncontrolling interests	396
Stock-based compensation	648	641	643	583	568	558
Balance, end of period	240,422	238,754	238,041	236,320	235,702	234,976	240,422	236,320		236,957
Accumulated Deficit
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(47,600)	(47,961)	(47,783)	(44,836)	(46,287)	(45,183)	(47,783)	(45,183)
Net income (loss)	991	361	(178)	(898)	1,451	(1,104)
Balance, end of period	(46,609)	(47,600)	(47,961)	(45,734)	(44,836)	(46,287)	(46,609)	(45,734)		(47,783)
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(6,365)	(6,637)	(4,862)	(3,130)	(733)	(1,972)	(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)	4,373	272	(1,775)	(4,110)	(2,397)	2,389
Balance, end of period	(1,992)	(6,365)	(6,637)	(7,240)	(3,130)	(733)	(1,992)	(7,240)		(4,862)
AXT, Inc. Stockholders' Equity
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	188,362	187,016	187,885	191,308	191,528	190,835	187,885	190,835
Common stock options exercised	624	72	441	35	232
Purchase of subsidiary shares from noncontrolling interest					(74)
Sale of subsidiary shares to noncontrolling interests	396
Reclassification out of accumulated other comprehensive income and non-controlling interest upon the deconsolidation of a subsidiary						(1,150)
Stock-based compensation	648	641	643	583	568	558
Net income (loss)	991	361	(178)	(898)	1,451	(1,104)
Other comprehensive income (loss)	4,373	272	(1,775)	(4,110)	(2,397)	2,389
Balance, end of period	195,394	188,362	187,016	186,918	191,308	191,528	195,394	186,918		187,885
Noncontrolling Interests
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	5,692	5,174	4,877	4,347	4,453	3,697	4,877	3,697
Purchase of subsidiary shares from noncontrolling interest					(339)
Reclassification out of accumulated other comprehensive income and non-controlling interest upon the deconsolidation of a subsidiary						533
Net income (loss)	490	598	395	403	287	81
Net dividend declared by joint ventures		(89)
Other comprehensive income (loss)	316	9	(98)	(214)	(54)	142
Balance, end of period	$ 6,498	$ 5,692	$ 5,174	$ 4,536	$ 4,347	$ 4,453	$ 6,498	$ 4,536		$ 4,877