REVENUE - Contract Balances (Details) - USD ($) $ in Thousands	6 Months Ended
	Mar. 31, 2021	Sep. 30, 2020
Revenue from Contract with Customer [Abstract]
Contract costs and recognized income not yet billed, net of progress payments	$ 75,000	$ 84,426
Increase (decrease) in contract assets balance	(9,426)
Contract costs and recognized income not yet billed, noncurrent	7,700	7,500
Contract liabilities	22,964	$ 24,386
Increase (decrease) in contract liabilities	$ (1,422)