Consolidated Statements of Income (Loss) - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Revenue	$ 592,571	$ 748,586	$ 742,354
Cost of revenue	391,855	509,489	512,024
Depreciation	22,185	21,800	22,456
Gross profit	178,531	217,297	207,874
Selling, general and administrative expenses	156,937	168,621	165,702
Bad debt provision for troubled customers, net of recoveries	0	3,038	650
Impairment charges	106,062	0	0
Pension withdrawal expense	0	848	5,886
Gain on sale of subsidiary	0	0	(2,384)
Research and engineering	2,892	3,045	3,310
Depreciation and amortization	13,520	16,733	11,957
Acquisition-related expense, net	337	875	532
Income (loss) from operations	(101,217)	24,137	22,221
Interest expense	12,955	13,698	7,950
Income (loss) before provision for income taxes	(114,172)	10,439	14,271
Provision (benefit) for income taxes	(14,706)	4,359	7,426
Net income (loss)	(99,466)	6,080	6,845
Less: net income (loss) attributable to noncontrolling interests, net of taxes	(5)	20	9
Net income (loss) attributable to Mistras Group, Inc.	$ (99,461)	$ 6,060	$ 6,836
Earnings (loss) per common share
Basic (in dollars per share)	$ (3.41)	$ 0.21	$ 0.24
Diluted (in dollars per share)	$ (3.41)	$ 0.21	$ 0.23
Weighted average common shares outstanding:
Basic (in shares)	29,147	28,740	28,406
Diluted (in shares)	29,147	29,046	29,427