REVENUES - Disaggregation of Revenues (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of revenues
Total revenues	$ 166,644	$ 180,492	$ 329,593	$ 352,220
Point in time
Disaggregation of revenues
Total revenues	73,938	86,828	152,047	172,440
Over time
Disaggregation of revenues
Total revenues	92,706	93,664	177,546	179,780
United States
Disaggregation of revenues
Total revenues	123,447	135,285	241,570	259,788
International
Disaggregation of revenues
Total revenues	43,197	45,207	88,023	92,432
Commercial
Disaggregation of revenues
Total revenues	111,827	129,238	230,555	258,582
U.S. Government
Disaggregation of revenues
Total revenues	54,817	51,254	99,038	93,638
Aerospace & Defense
Disaggregation of revenues
Total revenues	83,278	95,124	150,169	172,635
Aerospace & Defense | Point in time
Disaggregation of revenues
Total revenues	37,127	46,610	64,000	82,897
Aerospace & Defense | Over time
Disaggregation of revenues
Total revenues	46,151	48,514	86,169	89,738
Aerospace & Defense | United States
Disaggregation of revenues
Total revenues	73,037	81,458	130,849	146,164
Aerospace & Defense | International
Disaggregation of revenues
Total revenues	10,241	13,666	19,320	26,471
Aerospace & Defense | Commercial
Disaggregation of revenues
Total revenues	33,973	51,550	62,114	93,417
Aerospace & Defense | U.S. Government
Disaggregation of revenues
Total revenues	49,305	43,574	88,055	79,218
USG
Disaggregation of revenues
Total revenues	39,555	43,768	94,095	96,602
USG | Point in time
Disaggregation of revenues
Total revenues	27,563	32,209	69,931	72,524
USG | Over time
Disaggregation of revenues
Total revenues	11,992	11,559	24,164	24,078
USG | United States
Disaggregation of revenues
Total revenues	27,445	28,706	64,490	63,665
USG | International
Disaggregation of revenues
Total revenues	12,110	15,062	29,605	32,937
USG | Commercial
Disaggregation of revenues
Total revenues	38,549	43,736	92,414	95,971
USG | U.S. Government
Disaggregation of revenues
Total revenues	1,006	32	1,681	631
Test
Disaggregation of revenues
Total revenues	43,811	41,600	85,329	82,983
Test | Point in time
Disaggregation of revenues
Total revenues	9,248	8,009	18,116	17,019
Test | Over time
Disaggregation of revenues
Total revenues	34,563	33,591	67,213	65,964
Test | United States
Disaggregation of revenues
Total revenues	22,965	25,121	46,231	49,959
Test | International
Disaggregation of revenues
Total revenues	20,846	16,479	39,098	33,024
Test | Commercial
Disaggregation of revenues
Total revenues	39,305	33,952	76,027	69,194
Test | U.S. Government
Disaggregation of revenues
Total revenues	$ 4,506	$ 7,648	$ 9,302	$ 13,789