OTHER COMPREHENSIVE INCOME (LOSS) - AOCI (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019
Class of Stock [Line Items]
Accumulated other comprehensive income (loss)	$ 785,341	$ 764,431	$ 740,047	$ 700,151	$ 508,455	$ 476,454	$ 494,693	$ 477,763
Foreign currency translation adjustments
Class of Stock [Line Items]
Accumulated other comprehensive income (loss)	(10,661)			(25,683)
Pension and other defined benefit plans
Class of Stock [Line Items]
Accumulated other comprehensive income (loss)	(42,402)			(46,598)
Cash flow hedges
Class of Stock [Line Items]
Accumulated other comprehensive income (loss)	1,643			189
Available-for-sale securities
Class of Stock [Line Items]
Accumulated other comprehensive income (loss)	(17)			0
Accumulated other comprehensive income (loss), attributable to parent
Class of Stock [Line Items]
Accumulated other comprehensive income (loss)	$ (51,437)	$ (54,176)	$ (58,951)	$ (72,092)	$ (65,207)	$ (73,909)	$ (59,075)	$ (65,916)