Business Segments (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2018 USD ($) segment	Mar. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)
Segment Reporting [Abstract]
Number of reportable segments	3		3
Segment Reporting Information [Line Items]
Revenue from product sales and services	$ 333,690	$ 299,901	$ 630,740	$ 579,773
Sales to external customers	333,690	299,901	630,740	579,773
Earnings before interest, taxes, depreciation and amortization (EBITDA)	46,288	30,854	75,998	43,959
Depreciation and amortization	20,480	18,873	40,363	37,520
Income (loss) from operations	25,808	11,981	35,635	6,439
Interest expense	(10,810)	(11,898)	(28,053)	(26,651)
Income (loss) before income taxes	14,998	83	7,582	(20,212)
Income tax benefit (expense)	(2,018)	4,812	2,393	11,907
Net income (loss)	12,980	4,895	9,975	(8,305)
Capital expenditures	16,413	13,566	31,670	27,229
Assets	1,499,859		1,499,859		$ 1,473,309
Goodwill	329,898		329,898		321,913
Industrial [Member]
Segment Reporting Information [Line Items]
Capital expenditures	11,554	10,765	20,502	21,764
Municipal [Member]
Segment Reporting Information [Line Items]
Capital expenditures	1,225	838	2,972	1,434
Product [Member]
Segment Reporting Information [Line Items]
Capital expenditures	1,139	1,596	2,321	2,581
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue from product sales and services	21,995	18,236	44,294	33,722
Intersegment Eliminations [Member] | Industrial [Member]
Segment Reporting Information [Line Items]
Revenue from product sales and services	2,690	1,048	5,764	2,347
Intersegment Eliminations [Member] | Municipal [Member]
Segment Reporting Information [Line Items]
Revenue from product sales and services	9,214	5,140	16,621	11,895
Intersegment Eliminations [Member] | Product [Member]
Segment Reporting Information [Line Items]
Revenue from product sales and services	10,091	12,048	21,909	19,480
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue from product sales and services	355,684	318,138	675,035	613,496
Operating Segments [Member] | Industrial [Member]
Segment Reporting Information [Line Items]
Revenue from product sales and services	183,949	161,505	352,217	308,510
Sales to external customers	181,259	160,457	346,453	306,163
Earnings before interest, taxes, depreciation and amortization (EBITDA)	41,713	35,373	81,840	67,374
Depreciation and amortization	10,709	9,528	20,854	18,440
Income (loss) from operations	31,004	25,845	60,986	48,934
Assets	480,435		480,435		461,471
Goodwill	130,186		130,186		128,190
Operating Segments [Member] | Municipal [Member]
Segment Reporting Information [Line Items]
Revenue from product sales and services	73,498	68,694	142,811	138,080
Sales to external customers	64,284	63,554	126,190	126,185
Earnings before interest, taxes, depreciation and amortization (EBITDA)	9,758	8,526	16,350	17,088
Depreciation and amortization	1,777	2,050	3,577	4,125
Income (loss) from operations	7,981	6,476	12,773	12,963
Assets	139,195		139,195		155,698
Goodwill	9,961		9,961		9,865
Operating Segments [Member] | Product [Member]
Segment Reporting Information [Line Items]
Revenue from product sales and services	98,237	87,939	180,007	166,906
Sales to external customers	88,147	75,890	158,097	147,425
Earnings before interest, taxes, depreciation and amortization (EBITDA)	22,972	16,956	33,646	31,855
Depreciation and amortization	3,087	3,105	6,076	6,639
Income (loss) from operations	19,885	13,851	27,570	25,216
Assets	535,792		535,792		513,941
Goodwill	189,751		189,751		183,858
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Earnings before interest, taxes, depreciation and amortization (EBITDA)	(28,155)	(30,001)	(55,838)	(72,358)
Depreciation and amortization	4,907	4,190	9,856	8,316
Income (loss) from operations	(33,062)	(34,191)	(65,694)	(80,674)
Capital expenditures	2,495	$ 367	5,875	$ 1,450
Assets	$ 344,437		$ 344,437		$ 342,199