Net Income (Loss) Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Numerator:
Net income (loss) attributable to AXT, Inc.	$ (898)	$ 3,939	$ (551)	$ 10,715
Less: Preferred stock dividends	(44)	(44)	(132)	(132)
Net income (loss) available to common stockholders	$ (942)	$ 3,895	$ (683)	$ 10,583
Weighted-average number of common shares outstanding:
Denominator for basic net income (loss) per share - weighted-average common shares	39,466,000	39,008,000	39,438,000	39,000,000
Effect of dilutive securities:
Denominator for dilutive net income (loss) per common shares	39,466,000	40,331,000	39,438,000	40,320,000
Net income (loss) attributable to AXT, Inc. per common share:
Basic	$ (0.02)	$ 0.10	$ (0.02)	$ 0.27
Diluted	$ (0.02)	$ 0.10	$ (0.02)	$ 0.26
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)	2,523,000	186,000	2,523,000	186,000
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)	698,000	34,000	698,000	239,000
Options
Effect of dilutive securities:
Effect of dilutive securities (in shares)		1,207,000		1,196,000
Restricted Stock Awards
Effect of dilutive securities:
Effect of dilutive securities (in shares)		116,000		124,000