Net Income (Loss) Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Numerator:
Net income attributable to AXT, Inc.	$ 3,800	$ 991	$ 11,610	$ 1,174
Less: Preferred stock dividends	(44)	(44)	(132)	(132)
Net income available to common stockholders	$ 3,756	$ 947	$ 11,478	$ 1,042
Weighted-average number of common shares outstanding:
Denominator for basic net income per share - weighted-average common shares	41,460,000	40,152,000	41,237,000	39,978,000
Effect of dilutive securities:
Denominator for dilutive net income per common shares	42,678,000	40,979,000	42,710,000	40,688,000
Net income attributable to AXT, Inc. per common share:
Basic	$ 0.09	$ 0.02	$ 0.28	$ 0.03
Diluted	$ 0.09	$ 0.02	$ 0.27	$ 0.03
Preferred stock, shares issued (in shares)	883,000		883,000		883,000
Preferred stock, shares outstanding (in shares)	883,000		883,000		883,000
Preferred stock, par value (in dollars per share)	$ 0.001		$ 0.001		$ 0.001
Preferred stock, value	$ 3,532		$ 3,532		$ 3,532
Cumulative annual dividend rate (as a percent)			5.00%		5.00%
Liquidation preference over common stock (in dollars per share)	$ 4		$ 4		$ 4
Options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from diluted net income (loss) per share as the impact is anti-dilutive (in shares)	124,000	1,036,000	21,000	1,298,000
Restricted Stock Awards
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from diluted net income (loss) per share as the impact is anti-dilutive (in shares)	138,000	218,000	112,000	347,000
Options
Effect of dilutive securities:
Effect of dilutive securities (in shares)	691,000	575,000	887,000	510,000
Restricted Stock Awards
Effect of dilutive securities:
Effect of dilutive securities (in shares)	527,000	252,000	586,000	200,000

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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