CONSOLIDATING GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Condensed Consolidating Statements of Operations and Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Condensed Income Statements, Captions [Line Items]
Revenue	$ 548,438	$ 510,522
Cost of goods and services	398,517	367,476
Gross profit	149,921	143,046
Selling, general and administrative expenses	117,798	113,754
Income from operations	32,123	29,292
Other income (expense)
Interest income (expense), net	(15,950)	(16,331)
Other, net	778	1,004
Total other expense, net	(15,172)	(15,327)
Income before taxes	16,951	13,965
Provision (benefit) for income taxes	6,339	5,212
Income (loss) before equity in net income of subsidiaries	10,612	8,753
Equity in net income (loss) of subsidiaries	0	0
Net income	10,612	8,753
Comprehensive income (loss)	17,453	3,303
Reportable Legal Entities | Parent Company
Condensed Income Statements, Captions [Line Items]
Revenue	0	0
Cost of goods and services	0	0
Gross profit	0	0
Selling, general and administrative expenses	6,018	5,060
Income from operations	(6,018)	(5,060)
Other income (expense)
Interest income (expense), net	(6,324)	(6,307)
Other, net	(97)	(262)
Total other expense, net	(6,421)	(6,569)
Income before taxes	(12,439)	(11,629)
Provision (benefit) for income taxes	(4,395)	(3,535)
Income (loss) before equity in net income of subsidiaries	(8,044)	(8,094)
Equity in net income (loss) of subsidiaries	18,656	16,847
Net income	10,612	8,753
Comprehensive income (loss)	17,453	3,303
Reportable Legal Entities | Guarantor Companies
Condensed Income Statements, Captions [Line Items]
Revenue	441,392	419,244
Cost of goods and services	327,213	309,097
Gross profit	114,179	110,147
Selling, general and administrative expenses	87,848	84,976
Income from operations	26,331	25,171
Other income (expense)
Interest income (expense), net	(9,673)	(9,130)
Other, net	(1,351)	687
Total other expense, net	(11,024)	(8,443)
Income before taxes	15,307	16,728
Provision (benefit) for income taxes	5,727	5,974
Income (loss) before equity in net income of subsidiaries	9,580	10,754
Equity in net income (loss) of subsidiaries	9,196	6,050
Net income	18,776	16,804
Comprehensive income (loss)	18,776	53,569
Reportable Legal Entities | Non-Guarantor Companies
Condensed Income Statements, Captions [Line Items]
Revenue	117,039	98,240
Cost of goods and services	81,678	65,702
Gross profit	35,361	32,538
Selling, general and administrative expenses	24,024	23,817
Income from operations	11,337	8,721
Other income (expense)
Interest income (expense), net	47	(894)
Other, net	2,699	1,041
Total other expense, net	2,746	147
Income before taxes	14,083	8,868
Provision (benefit) for income taxes	5,007	2,775
Income (loss) before equity in net income of subsidiaries	9,076	6,093
Equity in net income (loss) of subsidiaries	9,580	10,754
Net income	18,656	16,847
Comprehensive income (loss)	18,656	(4,551)
Elimination
Condensed Income Statements, Captions [Line Items]
Revenue	(9,993)	(6,962)
Cost of goods and services	(10,374)	(7,323)
Gross profit	381	361
Selling, general and administrative expenses	(92)	(99)
Income from operations	473	460
Other income (expense)
Interest income (expense), net	0	0
Other, net	(473)	(462)
Total other expense, net	(473)	(462)
Income before taxes	0	(2)
Provision (benefit) for income taxes	0	(2)
Income (loss) before equity in net income of subsidiaries	0	0
Equity in net income (loss) of subsidiaries	(37,432)	(33,651)
Net income	(37,432)	(33,651)
Comprehensive income (loss)	$ (37,432)	$ (49,018)