REVENUES - Disaggregation of Revenues (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of revenues
Total revenues	$ 172,665	$ 178,259	$ 524,885	$ 512,867
Point in time
Disaggregation of revenues
Total revenues	73,364		245,106
Over time
Disaggregation of revenues
Total revenues	99,301		279,779
United States
Disaggregation of revenues
Total revenues	119,416		379,194
International
Disaggregation of revenues
Total revenues	53,249		145,691
Commercial
Disaggregation of revenues
Total revenues	115,962		374,459
U.S. Government
Disaggregation of revenues
Total revenues	56,703		150,426
Aerospace & Defense
Disaggregation of revenues
Total revenues	84,072	83,067	256,707	228,769
Aerospace & Defense | Point in time
Disaggregation of revenues
Total revenues	36,461		118,660
Aerospace & Defense | Over time
Disaggregation of revenues
Total revenues	47,611		138,047
Aerospace & Defense | United States
Disaggregation of revenues
Total revenues	73,476		219,640
Aerospace & Defense | International
Disaggregation of revenues
Total revenues	10,596		37,067
Aerospace & Defense | Commercial
Disaggregation of revenues
Total revenues	37,485		130,817
Aerospace & Defense | U.S. Government
Disaggregation of revenues
Total revenues	46,587		125,890
Test
Disaggregation of revenues
Total revenues	46,016	42,298	128,999	126,459
Test | Point in time
Disaggregation of revenues
Total revenues	9,138		26,157
Test | Over time
Disaggregation of revenues
Total revenues	36,878		102,842
Test | United States
Disaggregation of revenues
Total revenues	19,441		69,399
Test | International
Disaggregation of revenues
Total revenues	26,575		59,600
Test | Commercial
Disaggregation of revenues
Total revenues	38,737		107,931
Test | U.S. Government
Disaggregation of revenues
Total revenues	7,279		21,068
USG
Disaggregation of revenues
Total revenues	42,577	$ 52,894	139,179	$ 157,639
USG | Point in time
Disaggregation of revenues
Total revenues	27,765		100,289
USG | Over time
Disaggregation of revenues
Total revenues	14,812		38,890
USG | United States
Disaggregation of revenues
Total revenues	26,499		90,155
USG | International
Disaggregation of revenues
Total revenues	16,078		49,024
USG | Commercial
Disaggregation of revenues
Total revenues	39,740		135,711
USG | U.S. Government
Disaggregation of revenues
Total revenues	$ 2,837		$ 3,468