Stockholders' Equity (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Oct. 27, 2014
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period			$ 188,317,000
Common stock options exercised			178,000
Purchase of subsidiary shares from noncontrolling interest			(1,355,000)
Stock-based compensation			931,000
Net income	$ 4,551,000	$ 1,798,000	7,741,000	$ 2,223,000
Other comprehensive loss	(4,978,000)	845,000	(2,279,000)	1,197,000
Balance, end of period	193,533,000		193,533,000		$ 188,317,000
Reclassification adjustment from AOCI	0	$ 0	0	$ 0
Stock repurchase program, authorized amount								$ 5,000,000
Shares repurchased (in shares)					0	0	908,000
Average price of shares repurchased (in dollars per share)							$ 2.52
Total purchase price							$ 2,300,000
Stock repurchase program remaining authorized repurchase amount	2,700,000		2,700,000
Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period			3,532,000
Balance, end of period	3,532,000		3,532,000		$ 3,532,000
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period			39,000
Balance, end of period	39,000		39,000		39,000
Additional Paid-In Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period			231,679,000
Common stock options exercised			178,000
Purchase of subsidiary shares from noncontrolling interest			196,000
Stock-based compensation			931,000
Balance, end of period	232,984,000		232,984,000		231,679,000
Accumulated Deficit
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period			(54,837,000)
Net income			6,776,000
Balance, end of period	(48,061,000)		(48,061,000)		(54,837,000)
Accumulated Other Comprehensive Income
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period			3,407,000
Other comprehensive loss			(2,235,000)
Balance, end of period	1,172,000		1,172,000		3,407,000
AXT, Inc. Stockholders' Equity
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period			183,820,000
Common stock options exercised			178,000
Purchase of subsidiary shares from noncontrolling interest			196,000
Stock-based compensation			931,000
Net income			6,776,000
Other comprehensive loss			(2,235,000)
Balance, end of period	189,666,000		189,666,000		183,820,000
Noncontrolling Interests
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period			4,497,000
Purchase of subsidiary shares from noncontrolling interest			(1,551,000)
Net income			965,000
Other comprehensive loss			(44,000)
Balance, end of period	$ 3,867,000		$ 3,867,000		$ 4,497,000