Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended		12 Months Ended	36 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2015	Dec. 31, 2018	Oct. 27, 2014
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	$ 195,655	$ 195,981	$ 194,532	$ 193,533	$ 194,739	$ 188,317	$ 194,532	$ 188,317
Common stock options exercised	35	232		86	112	66
Purchase of subsidiary shares from noncontrolling interest		(413)		(252)	(1,355)
Reclassification out of accumulated other comprehensive income and non-controlling interest upon the deconsolidation of a subsidiary			(617)				(617)
Stock-based compensation	583	568	558	461	464	467
Net income (loss)	(495)	1,738	(1,023)	4,147	4,551	3,190	220	11,888
Other comprehensive income (loss)	(4,324)	(2,451)	2,531	(3,257)	(4,978)	2,699
Balance, end of period	191,454	195,655	195,981	194,718	193,533	194,739	191,454	194,718		$ 194,532
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	40	40	40	39	39	39	40	39
Balance, end of period	40	40	40	39	39	39	40	39		40
Additional Paid-In Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	235,702	234,976	234,418	232,984	232,212	231,679	234,418	231,679
Common stock options exercised	35	232		86	112	66
Purchase of subsidiary shares from noncontrolling interest		(74)		(9)	196
Stock-based compensation	583	568	558	461	464	467
Balance, end of period	236,320	235,702	234,976	233,522	232,984	232,212	236,320	233,522		234,418
Accumulated Deficit
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(44,836)	(46,287)	(45,183)	(48,061)	(51,962)	(54,837)	(45,183)	(54,837)
Net income (loss)	(898)	1,451	(1,104)	3,939	3,901	2,875
Balance, end of period	(45,734)	(44,836)	(46,287)	(44,122)	(48,061)	(51,962)	(45,734)	(44,122)		(45,183)
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(3,130)	(733)	(1,972)	1,172	5,846	3,407	(1,972)	3,407
Reclassification out of accumulated other comprehensive income and non-controlling interest upon the deconsolidation of a subsidiary			(1,150)
Other comprehensive income (loss)	(4,110)	(2,397)	2,389	(3,018)	(4,674)	2,439
Balance, end of period	(7,240)	(3,130)	(733)	(1,846)	1,172	5,846	(7,240)	(1,846)		(1,972)
AXT, Inc. Stockholders' Equity
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	191,308	191,528	190,835	189,666	189,667	183,820	190,835	183,820
Common stock options exercised	35	232		86	112	66
Purchase of subsidiary shares from noncontrolling interest		(74)		(9)	196
Reclassification out of accumulated other comprehensive income and non-controlling interest upon the deconsolidation of a subsidiary			(1,150)
Stock-based compensation	583	568	558	461	464	467
Net income (loss)	(898)	1,451	(1,104)	3,939	3,901	2,875
Other comprehensive income (loss)	(4,110)	(2,397)	2,389	(3,018)	(4,674)	2,439
Balance, end of period	186,918	191,308	191,528	191,125	189,666	189,667	186,918	191,125		190,835
Noncontrolling Interests
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	4,347	4,453	3,697	3,867	5,072	4,497	3,697	4,497
Purchase of subsidiary shares from noncontrolling interest		(339)		(243)	(1,551)
Reclassification out of accumulated other comprehensive income and non-controlling interest upon the deconsolidation of a subsidiary			533
Net income (loss)	403	287	81	208	650	315
Other comprehensive income (loss)	(214)	(54)	142	(239)	(304)	260
Balance, end of period	$ 4,536	$ 4,347	$ 4,453	$ 3,593	$ 3,867	$ 5,072	$ 4,536	$ 3,593		$ 3,697