Investments in Privately Held Companies - Minority Investment Entities (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) entity	Dec. 31, 2017 USD ($) company	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Summarized income information of all the minority investment entities that are not consolidated and accounted for under the equity method [Abstract]
Net (loss) income			$ (900,000)	$ (1,200,000)	$ (43,000)
Minority investment entities
Minority investments not consolidated accounted for under equity method	7	7
AXT, Inc. Stockholders' Equity
Summarized income information of all the minority investment entities that are not consolidated and accounted for under the equity method [Abstract]
Net revenue			6,152,000	6,124,000	9,007,000
Gross profit (loss)			482,000	(511,000)	1,975,000
Operating loss			(938,000)	(1,724,000)	588,000
Net (loss) income			(1,381,000)	(1,891,000)	558,000
Seven Minority Investments
Minority investment entities
Entity income (loss) excluding impairment			(1,400,000)	(2,000,000)	462,000
Minority Investment Entities
Summarized income information of all the minority investment entities that are not consolidated and accounted for under the equity method [Abstract]
Net revenue			24,053,000	23,266,000	35,732,000
Gross profit (loss)			1,739,000	(2,191,000)	8,429,000
Operating loss			(3,676,000)	(6,869,000)	2,965,000
Net (loss) income			(4,798,000)	(7,428,000)	2,850,000
Summarized balance sheet information of all the minority investment entities that are not consolidated and accounted for under the equity method [Abstract]
Current assets	$ 33,415,000	$ 33,415,000	33,415,000	31,672,000
Noncurrent assets	28,964,000	28,964,000	28,964,000	29,701,000
Current liabilities	27,274,000	27,274,000	27,274,000	23,412,000
Noncurrent liabilities	$ 180,000	$ 180,000	180,000	406,000
Minority investment entities
Dividends received			0	0	$ 305,000
Undistributed retained earnings			$ 3,600,000	$ 4,900,000