Net Income Per Share (Details) - USD ($)	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Numerator:
Net income attributable to AXT, Inc.	$ 3,939,000	$ 4,419,000	$ 10,715,000	$ 7,014,000
Less: Preferred stock dividends	(44,000)	(44,000)	(132,000)	(132,000)
Net income available to common stockholders	$ 3,895,000	$ 4,375,000	$ 10,583,000	$ 6,882,000
Weighted-average number of common shares outstanding:
Denominator for basic net income per share - weighted-average common shares (in shares)	39,008,000	38,499,000	39,000,000	36,999,000
Effect of dilutive securities:
Denominator for dilutive net income per common share (in shares)	40,331,000	40,095,000	40,320,000	38,469,000
Net income attributable to AXT, Inc. per common share:
Basic (in dollars per share)	$ 0.10	$ 0.11	$ 0.27	$ 0.19
Diluted (in dollars per share)	$ 0.10	$ 0.11	$ 0.26	$ 0.18
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	$ 3,532,000		$ 3,532,000		$ 3,532,000
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 per share as the impact is anti-dilutive (in shares)	186,000	68,000	186,000	573,000
Restricted Stock Awards
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from diluted net income per share as the impact is anti-dilutive (in shares)	34,000	34,000	239,000	11,000
Options
Effect of dilutive securities:
Effect of dilutive securities (in shares)	1,207,000	1,416,000	1,196,000	1,283,000
Restricted Stock Awards
Effect of dilutive securities:
Effect of dilutive securities (in shares)	116,000	180,000	124,000	187,000

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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