Atkore (ATKR) 10-K 19 Nov 20 2020 FY Annual report Financial data

Cover

Cover - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Nov. 13, 2020	Mar. 27, 2020
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Sep. 30, 2020
Document Transition Report	false
Entity File Number	001-37793
Entity Registrant Name	Atkore International Group Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	90-0631463
Entity Address, Address Line One	16100 South Lathrop Avenue
Entity Address, City or Town	Harvey
Entity Address, State or Province	IL
Entity Address, Postal Zip Code	60426
City Area Code	708
Local Phone Number	339-1610
Title of 12(b) Security	Common stock, par value $0.01 per share
Trading Symbol	ATKR
Security Exchange Name	NYSE
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 965.1
Entity Common Stock, Shares Outstanding		47,409,886
Documents Incorporated by Reference	Portions of the registrant's proxy statement to be filed with the United States Securities and Exchange Commission in connection with the registrant's 2021 annual meeting of stockholders (the "Proxy Statement") are incorporated by reference into Part III hereof. Such Proxy Statement will be filed within 120 days of the registrant's fiscal year ended September 30, 2020 .
Entity Central Index Key	0001666138
Document Fiscal Year Focus	2020
Amendment Flag	false
Document Fiscal Period Focus	FY
Current Fiscal Year End Date	--09-30

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Net sales	$ 1,765,421	$ 1,916,538	$ 1,835,139
Cost of sales	1,274,107	1,419,338	1,397,055
Gross profit	491,314	497,200	438,084
Selling, general and administrative	219,496	240,660	226,282
Intangible asset amortization	32,262	32,876	32,104
Operating income	239,556	223,664	179,698
Interest expense, net	40,062	50,473	40,694
Loss on extinguishment of debt	273	0	0
Other income, net	(2,777)	(11,478)	(27,348)
Income before income taxes	201,998	184,669	166,352
Income tax expense	49,696	45,618	29,707
Net income	$ 152,302	$ 139,051	$ 136,645
Net income per share
Basic (in dollars per share)	$ 3.15	$ 2.91	$ 2.59
Diluted (in dollars per share)	$ 3.10	$ 2.83	$ 2.48

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 152,302	$ 139,051	$ 136,645
Change in foreign currency translation adjustment	6,087	(7,490)	(2,853)
Change in unrecognized (loss) income related to pension benefit plans	(6,943)	(15,437)	4,397
Total other comprehensive (loss) income	(856)	(22,927)	1,544
Comprehensive income	$ 151,446	$ 116,124	$ 138,189

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Current Assets:
Cash and cash equivalents	$ 284,471	$ 123,415
Accounts receivable, less allowance for doubtful accounts of $3,168 and $2,608, respectively	298,242	315,353
Inventories, net	199,095	226,090
Prepaid expenses and other current assets	46,868	34,679
Total current assets	828,676	699,537
Property, plant and equipment, net	243,891	260,703
Intangible assets, net	255,349	285,684
Goodwill	188,239	186,231
Right-of-use assets, net	38,692
Deferred income taxes	687	577
Other long-term assets	2,991	4,263
Total Assets	1,558,525	1,436,995
Current Liabilities:
Accounts payable	142,601	150,681
Income tax payable	1,360	2,157
Accrued compensation and employee benefits	32,836	35,770
Customer liabilities	35,802	44,983
Lease obligations	15,786
Other current liabilities	47,785	53,943
Total current liabilities	276,170	287,534
Long-term debt	803,736	845,317
Long-term lease obligations	24,143
Deferred income taxes	22,525	19,986
Other long-term tax liabilities	1,619	3,669
Pension liabilities	40,023	34,509
Other long-term liabilities	11,899	13,044
Total Liabilities	1,180,115	1,204,059
Equity:
Common stock, $0.01 par value, 1,000,000,000 shares authorized, 47,407,023 and 46,955,163 shares issued and outstanding, respectively	475	471
Treasury stock, held at cost, 260,900 and 260,900 shares, respectively	(2,580)	(2,580)
Additional paid-in capital	487,223	477,139
Accumulated deficit	(64,154)	(200,396)
Accumulated other comprehensive loss	(42,554)	(41,698)
Total Equity	378,410	232,936
Total Liabilities and Equity	$ 1,558,525	$ 1,436,995

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Statement of Financial Position [Abstract]
Accounts receivable, less allowance for doubtful accounts of $3,168 and $2,608, respectively	$ 3,168	$ 2,608
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (shares)	1,000,000,000	1,000,000,000
Common stock, shares issued (shares)	47,407,023	46,955,163
Common stock, shares outstanding (shares)	47,407,023	46,955,163
Treasury stock (shares)	260,900	260,900

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Operating activities
Net income	$ 152,302	$ 139,051	$ 136,645
Adjustments to reconcile net income to net cash provided by operating activities
Gain on sale of a business	0	0	(27,575)
Depreciation and amortization	74,470	72,347	66,890
Amortization of debt issuance costs and original issue discount	1,876	1,804	1,542
Deferred income taxes	4,483	(796)	(9,008)
Loss on extinguishment of debt	273	0	0
Provision for losses on accounts receivable and inventory	5,014	4,656	7,241
Stock-based compensation expense	13,064	11,798	14,664
Amortization of right-of-use assets	14,803
Loss on disposal of property, plant & equipment, net	3,001	0	0
Gain on purchase of business	0	(7,384)	0
Other adjustments to net income	(844)	(1,938)	2,023
Changes in operating assets and liabilities, net of effects from acquisitions
Accounts receivable	16,920	6,026	(44,419)
Inventories	24,642	9,002	(36,528)
Prepaid expenses and other current assets	(11,164)	(3,054)	(1,762)
Accounts payable	(5,835)	(21,981)	31,667
Income taxes	(6,261)	4,511	(3,179)
Accrued and other liabilities	(32,942)	(2,782)	7,243
Other, net	(5,040)	(1,566)	259
Net cash provided by operating activities	248,762	209,694	145,703
Investing activities
Capital expenditures	(33,770)	(34,860)	(38,501)
Insurance proceeds for properties, plant and equipment	2,337	0	0
Proceeds from sale of properties, plant and equipment	3,920	80	349
Proceeds from divestiture of business	0	0	42,631
Acquisitions of businesses, net of cash acquired	0	(97,999)	(3,467)
Other, net	0	(322)	1,502
Net cash (used for) provided by investing activities	(27,513)	(133,101)	2,514
Financing activities
Borrowings under credit facility	0	39,000	358,000
Repayments under credit facility	0	(39,000)	(443,000)
Repayments of short-term debt	0	(20,980)	(8,150)
Issuance of long-term debt	0	0	426,217
Repayments of long-term debt	(40,000)	(40,000)	(1,217)
Issuance of common stock, net of taxes withheld	(2,972)	7,374	20,110
Repurchase of common stock	(15,011)	(24,419)	(411,775)
Payments for debt financing costs and fees	(3,204)	0	(5,955)
Other, net	8	(155)	(161)
Net cash used for financing activities	(61,179)	(78,180)	(65,931)
Effects of foreign exchange rate changes on cash and cash equivalents	986	(1,660)	(1,342)
Increase (decrease) in cash and cash equivalents	161,056	(3,247)	80,944
Cash and cash equivalents at beginning of period	123,415	126,662	45,718
Cash and cash equivalents at end of period	284,471	123,415	126,662
Supplementary Cash Flow information
Interest paid	38,791	49,879	39,898
Income taxes paid, net of refunds	50,993	38,698	41,601
Capital expenditures, not yet paid	$ 1,278	$ 3,719	$ 916

CONSOLIDATED STATEMENTS OF SHAR

CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands		Total	Cumulative Effect, Period of Adoption, Adjustment	Common Stock	Treasury Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated DeficitCumulative Effect, Period of Adoption, Adjustment	Accumulated Other Comprehensive Income (Loss)
Beginning balance (shares) at Sep. 30, 2017				63,305
Beginning balance at Sep. 30, 2017		$ 360,871		$ 634	$ (2,580)	$ 423,232	$ (42,433)		$ (17,982)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		136,645					136,645
Other comprehensive income (loss)		1,544							1,544
Stock-based compensation		14,664				14,664
Issuance of common stock (shares)				2,795
Issuance of common stock		20,110		$ 28		20,082
Repurchase of common stock (shares)				(19,020)
Repurchase of common stock		(411,775)		$ (190)			(411,585)
Ending balance (shares) at Sep. 30, 2018				47,080
Ending balance at Sep. 30, 2018		122,059		$ 472	(2,580)	457,978	(317,373)		(16,438)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		139,051					139,051
Other comprehensive income (loss)		(22,927)							(22,927)
Reclassification of stranded tax benefits	[1]	0					2,333		(2,333)
Stock-based compensation		11,798				11,798
Issuance of common stock (shares)				1,105
Issuance of common stock		7,374		$ 11		7,363
Repurchase of common stock (shares)				(1,230)
Repurchase of common stock		(24,419)		$ (12)			(24,407)
Ending balance (shares) at Sep. 30, 2019				46,955
Ending balance at Sep. 30, 2019		$ 232,936	$ (1,053)	$ 471	(2,580)	477,139	(200,396)	$ (1,053)	(41,698)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Standards Update [Extensible List]		us-gaap:AccountingStandardsUpdate201602Member
Net income		$ 152,302					152,302
Other comprehensive income (loss)		(856)							(856)
Stock-based compensation		13,064				13,064
Issuance of common stock (shares)				846
Issuance of common stock		(2,972)		$ 8		(2,980)
Repurchase of common stock (shares)				(394)
Repurchase of common stock		(15,011)		$ (4)			(15,007)
Ending balance (shares) at Sep. 30, 2020				47,407
Ending balance at Sep. 30, 2020		$ 378,410		$ 475	$ (2,580)	$ 487,223	$ (64,154)		$ (42,554)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Standards Update [Extensible List]		us-gaap:AccountingStandardsUpdate201602Member
[1]	Due to the adoption of ASU 2018-02.

BASIS OF PRESENTATION AND SUMMA

BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	12 Months Ended
	Sep. 30, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	1. BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Basis of presentation Organization and Ownership Structure — Atkore International Group Inc. (the "Company" or "Atkore") is a leading manufacturer of Electrical Raceway products primarily for the non-residential construction and renovation markets and Mechanical Products & Solutions ("MP&S") for the construction and industrial markets. Electrical Raceway products form the critical infrastructure that enables the deployment, isolation and protection of a structure's electrical circuitry from the original power source to the final outlet. MP&S frame, support and secure component parts in a broad range of structures, equipment and systems in electrical, industrial and construction applications. The Company was incorporated in the State of Delaware on November 4, 2010. Atkore is the sole stockholder of Atkore International Holdings Inc. ("AIH"), which in turn is the sole stockholder of Atkore International, Inc. ("AII"). Holders of common stock are entitled to cast one vote for each share held of record on all matters submitted to a vote of the stockholders. Additionally, holders of common stock are entitled to receive, on a pro rata basis, dividends and distributions, if any, that the Company’s board of directors may declare out of legally available funds, subject to preferences that may be applicable to preferred stock, if any. Share Repurchase Program — In August 2017, the Company's board of directors approved a share repurchase program, under which the Company may repurchase up to an aggregate amount of $75.0 million of its outstanding stock. As of December 28, 2018, there were no authorized repurchases remaining under the August 2017 repurchase program. On February 5, 2019, the board of directors approved a share repurchase program, under which the Company may repurchase up to $50.0 million of its outstanding common stock. As of September 30, 2020, there were $35.0 million of authorized repurchases remaining. Basis of Presentation — The accompanying audited consolidated financial statements of the Company and all of its subsidiaries included herein have been prepared in accordance with accounting principles generally accepted in the United States of America ("GAAP"). The audited consolidated financial statements include the assets and liabilities used in operating the Company's business. All intercompany balances and transactions have been eliminated in consolidation. The results of companies acquired or disposed of are included in the audited consolidated financial statements from the effective date of acquisition or up to the date of disposal. Fiscal Periods — The Company has a fiscal year that ends on September 30. The Company's fiscal quarters typically end on the last Friday in December, March and June as it follows a 4-5-4 calendar. Use of Estimates — The preparation of the consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclose contingent assets and liabilities at the date of the consolidated financial statements and report the associated amounts of revenues and expenses. Actual results could differ materially from these estimates. Summary of significant accounting policies Revenue Recognition — The Company’s revenue arrangements primarily consist of a single performance obligation to transfer promised goods which is satisfied at a point in time when title, risks and rewards of ownership, and subsequently control have transferred to the customer. This generally occurs when the product is shipped to the customer, with an immaterial amount of transactions in which control transfers upon delivery. The Company primarily offers assurance-type standard warranties that do not represent separate performance obligations. The Company has certain arrangements that require it to estimate at the time of sale the amounts of variable consideration that should not be recorded as revenue as certain amounts are not expected to be collected from customers, as well as an estimate of the value of products to be returned. The Company principally relies on historical experience, specific customer agreements, and anticipated future trends to estimate these amounts at the time of sale and to reduce the transaction price. These arrangements include sales discounts and allowances, volume rebates, and returned goods. The Company has elected to utilize certain practical expedients available under GAAP. The Company records amounts billed to customers for reimbursement of shipping and handling costs within revenue. Shipping and handling costs associated with outbound freight after control over a product has transferred to a customer are accounted for as fulfillment costs and are included in cost of sales. Sales taxes and other usage-based taxes are excluded from revenue. The practical expedient not to disclose information about remaining performance obligations has also been elected as these obligations have an original duration of one year or less. The Company does not evaluate whether the selling price includes a financing interest component for contracts that are less than a year. The Company also expenses costs incurred to obtain a contract, primarily sales commissions, as all obligations will be settled in less than one year. The Company typically receives payment 30 to 60 days from the point it has satisfied the related performance obligation. See Note 20, ''Segment Information'' for revenue disaggregated by geography and product categories. Cost of Sales — The Company includes all costs directly related to the production of goods for sale in cost of sales in the statement of operations. These costs include direct material, direct labor, production related overheads, excess and obsolescence costs, lower of cost or market provisions, freight and distribution costs, and the depreciation and amortization of assets directly used in the production of goods for sale. Selling, General and Administrative Expenses — These amounts primarily include payroll-related expenses for both administrative and selling personnel, compensation expense from stock-based awards, restructuring-related charges, third-party professional services and transactional gains or losses for foreign currency transactions, excluding the foreign exchange exposure for intercompany loan transactions, which is included in Other income, net . Cash and Cash Equivalents — The Company considers all highly liquid investments with a maturity of three months or less, when purchased, to be cash equivalents. Accounts Receivable and Allowance for Doubtful Accounts — The Company carries its accounts receivable at their face amounts less an allowance for doubtful accounts. The allowance for doubtful accounts receivable reflects the best estimate of losses inherent in the Company's accounts receivable portfolio determined on the basis of historical experience, specific allowances for known troubled accounts and other available evidence. Inventories — Inventories are recorded at the lower of cost (primarily LIFO) or market value. The Company estimates losses for excess and obsolete inventory through an assessment of its net realizable value based on the aging of the inventory and an evaluation of the likelihood of recovering the inventory costs based on anticipated demand and selling price. See Note 12, ''Inventories, net.'' Property, Plant and Equipment — Property, plant and equipment, net, is recorded at cost less accumulated depreciation. Maintenance and repair expenditures are charged to expense when incurred. Depreciation is calculated using the straight-line method over the estimated useful lives of the related assets as follows: Buildings 4 to 40 years Building improvements 3 to 20 years Machinery and equipment 1 to 20 years Leasehold improvements Lesser of remaining term of the lease or useful life Software 2 to 10 years The internal and external costs incurred to develop internal use computer software during the application development stage of the implementation, including the design of the chosen path, are capitalized. Other costs, including expenses incurred during the preliminary project stage, training expenses, data conversion costs and expenses incurred in the post implementation stage are expensed in the period incurred. Capitalized costs are amortized ratably over the useful life of the software when the software becomes operational. Upgrades and enhancements to internal use software are capitalized only if the costs result in additional functionality. The Company does not plan to sell or market our internal use computer software to third parties. Long-Lived Asset and Finite - Lived Intangible Asset Impairments — The Company reviews long-lived assets, including property, plant and equipment and finite-lived intangible assets for impairment whenever events or changes in business circumstances indicate that the carrying amount of the asset may not be fully recoverable. The Company groups assets at the lowest level for which cash flows are separately identified in order to measure an impairment. Recoverability of an asset or asset group is first measured by a comparison of the carrying amount to its estimated undiscounted future cash flows expected to be generated by the asset or asset group. If the carrying amount exceeds its estimated undiscounted future cash flows, an impairment charge is recognized as the amount by which the carrying amount of the asset or asset group exceeds the estimated fair value. If impairment is determined to exist, any related impairment loss is calculated based on the estimated fair value. Impairment losses on assets to be disposed of or held for sale, if any, are based on the estimated proceeds to be received, less costs of disposal. The Company also considers potential impairment indicators associated with other finite-lived intangible assets, including its customer relationships, patents, and non-compete agreements. An impairment is recognized if the carrying value of an asset or asset group exceeds the estimated undiscounted future cash flows expected to result from the use of the asset or asset group and its eventual disposition. The Company's key customers are primarily wholesale and national distributors. The terms of these relationships are based on purchase orders and are not contractually based. Customer relationships are amortized on a straight-line basis over their useful lives, ranging from 6 to 14 years. The Company evaluates the appropriateness of remaining useful lives based on customer attrition rates. Other intangible assets are amortized on a straight-lined basis over their estimated useful lives, ranging from 1 to 20 years. The Company did not have a triggering event during fiscal 2020 , 2019 and 2018. Goodwill and Indefinite-Lived Intangible Asset Impairments — The Company assesses the recoverability of goodwill and indefinite-lived trade names on an annual basis in accordance with Accounting Standards Codification ("ASC") 350 "Intangibles - Goodwill and Other." The measurement date is the first day of the fourth fiscal quarter, or more frequently, if events or circumstances indicate that it is more likely than not that the fair value of a reporting unit or the respective indefinite-lived trade name is less than the carrying value. The Company can elect to perform a quantitative or qualitative test of impairment. For fiscal 2020, 2019 and 2018 Company performed a quantitative impairment assessment for goodwill. Additionally, the economic impacts of the COVID-19 outbreak were more substantial for the Company’s reporting unit related to its operations in Europe, Middle East, and Africa (EMEA), due to the cross-border transactions that occur within those operations, the cross-border interactions needed to fully realize the synergies associated with recent in-region acquisitions and the COVID-19 related closing of these borders. The Company deemed the economic related developments of the outbreak to be a triggering event for the EMEA reporting unit as of March 27, 2020 and performed an interim goodwill impairment test as of that date. The Company calculated the fair value of its six reporting units considering three valuation approaches: (a) the income approach; (b) the guideline public company method; and (c) the comparable transaction method. The income approach calculates the fair value of the reporting unit using a discounted cash flow approach. Internally forecasted future cash flows, which the Company believes reasonably approximate market participant assumptions, are discounted using a weighted average cost of capital (Discount Rate) developed for each reporting unit. The Discount Rate is developed using market observable inputs, as well as considering whether or not there is a measure of risk related to the specific reporting unit’s forecasted performance. The key uncertainties in these calculations are the assumptions used in determining the reporting unit’s forecasted future performance, including revenue growth and EBITDA margins, as well as the perceived risk associated with those forecasts, along with selecting representative market multiples. Fair value under the guideline public company method is determined for each reporting unit by applying market multiples for comparable public companies to the reporting unit’s financial results. Fair value under the comparable transaction method is determined based on exchange prices in actual transactions and on asking prices for controlling interests in public or private companies currently offered for sale by applying market multiples for comparable public companies to the unit’s financial results. The key uncertainties in the guideline public company method and the comparable transaction method calculations are the assumptions used in determining the reporting unit's comparable public companies, comparable transactions and the selection of the market multiples. The Company did no t record any goodwill impairments in fiscal 2020, 2019 or 2018. As noted above, ASC 350 also requires that the Company test the indefinite-lived intangible assets for impairment at least annually. Under ASC 350, if the carrying value of the indefinite-lived asset is higher than its fair value, then the asset is deemed to be impaired and the impairment charge is estimated as the excess carrying value over the fair value. The Company calculated the fair value of its indefinite-lived intangible assets using the income approach, specifically the relief-from-royalty method. The relief-from-royalty method is used to estimate the cost savings that accrue to the owner of an intangible asset who would otherwise have to pay royalties or license fees on revenues earned through the use of the asset. Internally forecasted revenues, which the Company believes reasonably approximate market participant assumptions, are multiplied by a royalty rate to arrive at the estimated net after tax cost savings. The royalty rate used in the analysis is based on an analysis of empirical, market-derived royalty rates for guideline intangible assets. The net after tax cost savings are discounted using the Discount Rate. The Discount Rate is developed using market observable inputs, as well as considering whether or not there is a measure of risk related to the specific indefinite lived intangible assets' forecasted performance. The key uncertainties in these calculations are the assumptions used in determining the revenue associated with each indefinite-lived intangible asset and the royalty rate. The Company did no t record any indefinite-lived asset impairments in fiscal 2020, 2019 or 2018. Fair Value Measurements — Authoritative guidance for fair value measurements establishes a three-level hierarchy that ranks the quality and reliability of information used in developing fair value estimates. The hierarchy gives the highest priority to quoted prices in active markets and the lowest priority to unobservable data. In cases where two or more levels of inputs are used to determine fair value, a financial instrument's level is determined based on the lowest level input that is considered significant to the fair value measurement in its entirety. The three levels of the fair value hierarchy are summarized as follows: Level 1-inputs are based upon quoted prices (unadjusted) in active markets for identical assets or liabilities which are accessible as of the measurement date. Level 2-inputs are based upon quoted prices for similar assets or liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, and model-derived valuations for the asset or liability that are derived principally from or corroborated by market data for which the primary inputs are observable, including forward interest rates, yield curves, credit risk and exchange rates. Level 3-inputs for the valuations are unobservable and are based on management's estimates of assumptions that market participants would use in pricing the asset or liability. The fair values are therefore determined using model-based techniques such as option pricing models and discounted cash flow models. Income Taxes and Uncertain Tax Positions — The Company accounts for income taxes under the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the consolidated financial statements. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and tax basis of assets and liabilities using enacted tax rates in effect for the year it is expected the differences will reverse. The effect of a change in tax rates on deferred tax assets and liabilities is recognized in income in the period of the enactment date. The Company periodically assesses the realizability of the deferred tax assets. In making this determination management considers all available evidence, both positive and negative, including earnings history, expectations of future taxable income and available tax planning strategies. A valuation allowance is recorded to reduce the Company's deferred tax assets to the amount that is considered more likely than not to be realized. Changes in the required valuation allowance are recorded in income in the period such determination is made. Certain tax positions may be considered uncertain requiring an assessment of whether an allowance should be recorded. Provisions for uncertain tax positions provide a recognition threshold based on an estimate of whether it is more likely than not that a position will be sustained upon examination. The Company measures its uncertain tax positions as the largest amount of benefit that is greater than a 50% likelihood of being realized upon examination. Interest and penalties related to unrecognized tax benefits are recorded as a component of income tax expense. See Note 9, ''Income Taxes.'' Leases — For fiscal 2019 and 2018, under ASC 840, all leases that are determined not to meet any of the capital lease criteria are classified as operating leases. Operating lease payments are recognized as an expense in the statement of operations on a straight-line basis over the lease term. For fiscal 2020, as a result of the adoption of ASC 842, the Company recognizes if an arrangement is a lease at the inception of the contract. The Company determines which party has the right to control an asset during the contract term and recognizes a Right of Use ("ROU") asset and lease obligations based on the present value of the future minimum lease payments over the term of the lease. Refer to Note 2 for further discussion of the Company’s accounting policy for leases. Translation of Foreign Currency — For the Company's non-U.S. subsidiaries that report in a functional currency other than United States dollars, assets and liabilities are translated into United States dollars using year-end exchange rates. Revenue and expenses are translated at the monthly average exchange rates in effect during the fiscal year. Foreign currency translation adjustments are included as a component of accumulated other comprehensive loss within the consolidated statements of comprehensive income . Recent Accounting Pronouncements A summary of recently adopted Accounting Standards Update ("ASU")s are as follows. Adoption dates are on the first day of the fiscal year indicated below, unless otherwise specified. Adopted Guidance ASU Description of ASU Impact to Atkore Note Adoption Date 2016-02 Leases (Topic 842) The ASU requires companies to use a "right of use" lease model that assumes that each lease creates an asset (the lessee's right to use the leased asset) and a liability (the future rent payment obligations), which should be reflected on a lessee's balance sheet to fairly represent the lease transaction and the lessee's related financial obligations with terms of more than 12 months. The Company adopted the guidance in the first quarter of 2020 using the modified retrospective method. See Note 2, "Leases" for further detail. 2 2020 2018-14 Disclosure Framework - Changes to the Disclosure Requirements for Defined Benefit Plans The ASU amends ASC 715 to add, remove and clarify disclosure requirements related to defined benefit pension and other postretirement plans. The Company elected to adopt the guidance early in the quarter ended September 30, 2020. There was no material impact to the consolidated financial statements as a result of the adoption of ASU 2018-14. See Note 5, "Post Retirement Benefits" for further detail. 5 2020 A summary of guidance not yet adopted are as follows: Guidance not yet adopted ASU Description of ASU Impact to Atkore Effective Date 2016-13 Financial Instruments - Credit Losses (Topic 326) The ASU adds to U.S. GAAP an impairment model (known as the current expected credit loss (CECL) model) that is based on expected losses rather than incurred losses. Under the new guidance, an entity recognizes as an allowance its estimate of expected credit losses, which the FASB believes will result in more timely recognition of such losses. The Company adopted this standard in the first quarter of 2021. The adoption of the standard did not have a material impact on the Company’s consolidated financial statements. The Company implemented the appropriate changes to business processes and controls to support recognition and disclosure under the new standard. 2021 2019-12, Simplifying the accounting for income taxes (Topics 740) The ASU eliminates certain existing exceptions related to the general approach in ASC 740 relating to franchise taxes, reducing complexity in the interim period accounting for year to date loss limitations and changes in tax laws, and clarifying the accounting for transactions outside of business combination that result in a step up in the tax basis of goodwill. Under evaluation. Based on procedures performed to date, the Company does not anticipate the adoption of this ASU to be material to the financial statements. 2022 2020-04, Reference rate reform Topic 848: Facilitation of the effects of reference rate reform on financial reporting The ASU addresses constituents’ concerns about certain accounting consequences that could result from the global markets’ anticipated transition away from the use of the London Interbank Offered Rate (LIBOR) and other interbank offered rates to alternative reference rates. Under evaluation. 2022

LEASES

LEASES	12 Months Ended
	Sep. 30, 2020
Leases [Abstract]
LEASES	2. LEASES On October 1, 2019, we adopted Topic 842 using the modified retrospective transition method. Topic 842 requires the recognition of lease assets and liabilities for operating leases, in addition to the finance lease assets and liabilities previously recorded on our consolidated balance sheets. Beginning on October 1, 2019, our consolidated financial statements are presented in accordance with the revised policies, while prior period amounts are not adjusted and continue to be reported in accordance with our historical policies. The modified retrospective transition method required the cumulative effect, if any, of initially applying the guidance to be recognized as an adjustment to our accumulated deficit as of our adoption date. As a result of adopting Topic 842 , we recognized additional operating lease assets and liabilities of $45,519 and $46,941 as of October 1, 2019. The discount rate primarily used to calculate that adjustment was the Company's incremental borrowing rate as of the adoption date, October 1, 2019, as a rate implicit in most contracts was not readily determinable. The Company recorded a cumulative effect adjustment of $1,053 to accumulated deficit, net of tax, as a result of the adoption. The Company elected the package of practical expedients permitted under the transition guidance within Topic 842, which allowed us to carry forward prior conclusions about lease identification, classification and initial direct costs for leases entered into prior to adoption of Topic 842. Additionally, for leases with a term of 12 months or less, the Company elected the short-term lease exemption, which allowed us to not recognize ROU assets or lease liabilities for qualifying leases existing at transition and new leases we may enter into in the future. Leases with an initial term of 12 months or less are classified as short-term leases and are not recorded on the consolidated balance sheets. The lease expense for short-term leases is recognized on a straight-line basis over the lease term. The Company engages in leasing transactions to meet the needs of the business. The Company leases certain manufacturing facilities, warehouses and distribution centers, office space, forklifts, vehicles and other machinery and equipment. The determination to lease, rather than purchase, an asset is primarily contingent upon capital requirements, duration of the forecasted business investment, and asset availability. The Company determines if an arrangement is a lease at inception and all arrangements deemed to be leases are subject to an assessment to determine the classification between finance and operating leases. The Company's significant assumptions and judgments in determining whether a contract is or contains a lease include establishing whether the supplier has the ability to use other assets to fulfill its service or whether the terms of the agreement enable the Company to control the use of a dedicated property, plant and equipment asset during the contract term. In the majority of the Company's contracts where it must identify whether a lease is present, it is readily determinable that the Company controls the use of the assets and obtains substantially all of the economic benefit during the term of the contract. In those contracts where identification is not readily determinable, the Company has determined that the supplier has either the ability to use another asset to provide the service or the terms of the contract give the supplier the rights to operate the asset at its discretion during the term of the contract, in which case the arrangement would not constitute a lease. Right-of-use assets and lease obligations are recognized based on the present value of the future minimum lease payments over the lease term as of the commencement date. The Company’s lease agreements have terms that include both lease and non-lease components. Lease component fees are included in the present value of future minimum lease payments. Conversely, non-lease components are not subject to capitalization and are expensed as incurred. Per Topic 842, the contractual interest rate is used to calculate the present value of the future minimum lease payments. However, the majority of the Company’s leases do not provide an implicit rate. Therefore, the Company's significant assumption and judgments in determining the discount rate include determining the incremental borrowing rate. The Company’s incremental borrowing rates are based on the term of the lease, the economic environment of the lease and the effect of collateralization. The valuation of the ROU asset also includes lease payments made in advance of the lease commencement date and initial direct costs incurred to secure the lease and is reduced for lease incentives. The lease terms include options to extend or terminate the lease when it is reasonably certain the Company will exercise the options. The Company has certain leasing agreements, related to leased vehicles available to our sales personnel, that contain guaranteed residual value terms, which are not expected to be triggered. The Company’s leasing portfolio does not contain any material restrictive covenants. Leases (in thousands) September 30, 2020 Assets Operating lease assets $ 37,666 Finance lease assets 2,999 Total right-of-use assets, gross $ 40,665 Less: accumulated amortization (1,973 ) Right-of-use assets, net $ 38,692 Liabilities Current liabilities: Current portion of operating lease liabilities $ 15,356 Current portion of finance lease liabilities 430 Lease obligations $ 15,786 Noncurrent liabilities: Operating lease liabilities $ 23,354 Finance lease liabilities 789 Long-term lease obligations $ 24,143 Total lease obligations $ 39,929 Lease Cost The following table summarizes lease costs by type of cost for the fiscal year ended September 30, 2020 . In the consolidated statements of operations, cost of sales and selling, general and administrative expenses included lease costs of $14,295 and $2,714 respectively. (in thousands) Fiscal year ended September 30, 2020 Consolidated Statement of Operations Classification Total Amortization of right-of-use assets $ 14,803 Interest on lease liabilities 41 Variable lease costs 714 Short term lease costs 1,451 Total lease costs $ 17,009 Maturity of Lease Liabilities The Company's maturity analysis of its lease liabilities as of fiscal year ended September 30, 2020 is as follows: (in thousands) Financing Leases Operating Leases 2021 $ 422 $ 12,902 2022 406 10,097 2023 332 7,228 2024 89 5,789 2025 3 3,451 2026 and after — 3,535 Total lease payments $ 1,252 $ 43,002 Less: Interest (33 ) (4,292 ) Present value of lease liabilities $ 1,219 $ 38,710 Total rental expense on all operating leases was $17,077 and $ 17,403 in fiscal 2019 , and 2018 , respectively. The following represents the Company's future minimum rental payments at September 30, 2019 , for agreements classified as operating leases under ASC 840 with non-cancelable terms in excess of one year: 2020 $ 13,526 2021 11,592 2022 8,666 2023 6,362 2024 5,097 2025 and thereafter 6,938 Total $ 52,181 Lease Term and Discount Rate Fiscal year ended September 30, 2020 Weighted-average remaining lease term (years) Operating leases 4.4 Finance leases 3.2 Weighted-average discount rate Operating leases 3.6 % Finance leases 3.1 % Other Information (in thousands) Fiscal year ended September 30, 2020 Cash paid for amounts included in the measurement of lease liabilities Operating cash flows from operating leases 12,939 Operating cash flows from finance leases 26 Financing cash flows from finance leases 500
LEASES	2. LEASES On October 1, 2019, we adopted Topic 842 using the modified retrospective transition method. Topic 842 requires the recognition of lease assets and liabilities for operating leases, in addition to the finance lease assets and liabilities previously recorded on our consolidated balance sheets. Beginning on October 1, 2019, our consolidated financial statements are presented in accordance with the revised policies, while prior period amounts are not adjusted and continue to be reported in accordance with our historical policies. The modified retrospective transition method required the cumulative effect, if any, of initially applying the guidance to be recognized as an adjustment to our accumulated deficit as of our adoption date. As a result of adopting Topic 842 , we recognized additional operating lease assets and liabilities of $45,519 and $46,941 as of October 1, 2019. The discount rate primarily used to calculate that adjustment was the Company's incremental borrowing rate as of the adoption date, October 1, 2019, as a rate implicit in most contracts was not readily determinable. The Company recorded a cumulative effect adjustment of $1,053 to accumulated deficit, net of tax, as a result of the adoption. The Company elected the package of practical expedients permitted under the transition guidance within Topic 842, which allowed us to carry forward prior conclusions about lease identification, classification and initial direct costs for leases entered into prior to adoption of Topic 842. Additionally, for leases with a term of 12 months or less, the Company elected the short-term lease exemption, which allowed us to not recognize ROU assets or lease liabilities for qualifying leases existing at transition and new leases we may enter into in the future. Leases with an initial term of 12 months or less are classified as short-term leases and are not recorded on the consolidated balance sheets. The lease expense for short-term leases is recognized on a straight-line basis over the lease term. The Company engages in leasing transactions to meet the needs of the business. The Company leases certain manufacturing facilities, warehouses and distribution centers, office space, forklifts, vehicles and other machinery and equipment. The determination to lease, rather than purchase, an asset is primarily contingent upon capital requirements, duration of the forecasted business investment, and asset availability. The Company determines if an arrangement is a lease at inception and all arrangements deemed to be leases are subject to an assessment to determine the classification between finance and operating leases. The Company's significant assumptions and judgments in determining whether a contract is or contains a lease include establishing whether the supplier has the ability to use other assets to fulfill its service or whether the terms of the agreement enable the Company to control the use of a dedicated property, plant and equipment asset during the contract term. In the majority of the Company's contracts where it must identify whether a lease is present, it is readily determinable that the Company controls the use of the assets and obtains substantially all of the economic benefit during the term of the contract. In those contracts where identification is not readily determinable, the Company has determined that the supplier has either the ability to use another asset to provide the service or the terms of the contract give the supplier the rights to operate the asset at its discretion during the term of the contract, in which case the arrangement would not constitute a lease. Right-of-use assets and lease obligations are recognized based on the present value of the future minimum lease payments over the lease term as of the commencement date. The Company’s lease agreements have terms that include both lease and non-lease components. Lease component fees are included in the present value of future minimum lease payments. Conversely, non-lease components are not subject to capitalization and are expensed as incurred. Per Topic 842, the contractual interest rate is used to calculate the present value of the future minimum lease payments. However, the majority of the Company’s leases do not provide an implicit rate. Therefore, the Company's significant assumption and judgments in determining the discount rate include determining the incremental borrowing rate. The Company’s incremental borrowing rates are based on the term of the lease, the economic environment of the lease and the effect of collateralization. The valuation of the ROU asset also includes lease payments made in advance of the lease commencement date and initial direct costs incurred to secure the lease and is reduced for lease incentives. The lease terms include options to extend or terminate the lease when it is reasonably certain the Company will exercise the options. The Company has certain leasing agreements, related to leased vehicles available to our sales personnel, that contain guaranteed residual value terms, which are not expected to be triggered. The Company’s leasing portfolio does not contain any material restrictive covenants. Leases (in thousands) September 30, 2020 Assets Operating lease assets $ 37,666 Finance lease assets 2,999 Total right-of-use assets, gross $ 40,665 Less: accumulated amortization (1,973 ) Right-of-use assets, net $ 38,692 Liabilities Current liabilities: Current portion of operating lease liabilities $ 15,356 Current portion of finance lease liabilities 430 Lease obligations $ 15,786 Noncurrent liabilities: Operating lease liabilities $ 23,354 Finance lease liabilities 789 Long-term lease obligations $ 24,143 Total lease obligations $ 39,929 Lease Cost The following table summarizes lease costs by type of cost for the fiscal year ended September 30, 2020 . In the consolidated statements of operations, cost of sales and selling, general and administrative expenses included lease costs of $14,295 and $2,714 respectively. (in thousands) Fiscal year ended September 30, 2020 Consolidated Statement of Operations Classification Total Amortization of right-of-use assets $ 14,803 Interest on lease liabilities 41 Variable lease costs 714 Short term lease costs 1,451 Total lease costs $ 17,009 Maturity of Lease Liabilities The Company's maturity analysis of its lease liabilities as of fiscal year ended September 30, 2020 is as follows: (in thousands) Financing Leases Operating Leases 2021 $ 422 $ 12,902 2022 406 10,097 2023 332 7,228 2024 89 5,789 2025 3 3,451 2026 and after — 3,535 Total lease payments $ 1,252 $ 43,002 Less: Interest (33 ) (4,292 ) Present value of lease liabilities $ 1,219 $ 38,710 Total rental expense on all operating leases was $17,077 and $ 17,403 in fiscal 2019 , and 2018 , respectively. The following represents the Company's future minimum rental payments at September 30, 2019 , for agreements classified as operating leases under ASC 840 with non-cancelable terms in excess of one year: 2020 $ 13,526 2021 11,592 2022 8,666 2023 6,362 2024 5,097 2025 and thereafter 6,938 Total $ 52,181 Lease Term and Discount Rate Fiscal year ended September 30, 2020 Weighted-average remaining lease term (years) Operating leases 4.4 Finance leases 3.2 Weighted-average discount rate Operating leases 3.6 % Finance leases 3.1 % Other Information (in thousands) Fiscal year ended September 30, 2020 Cash paid for amounts included in the measurement of lease liabilities Operating cash flows from operating leases 12,939 Operating cash flows from finance leases 26 Financing cash flows from finance leases 500

ACQUISITIONS

ACQUISITIONS	12 Months Ended
	Sep. 30, 2020
Business Combinations [Abstract]
ACQUISITIONS	3. ACQUISITIONS From time to time, the Company enters into strategic acquisitions in an effort to better service existing customers and to attain new customers. The Company had no acquisitions in the year ended September 30, 2020. Fiscal 2019 On August 21, 2019, Atkore Plastic Pipe Corporation, a wholly-owned subsidiary of the Company acquired the assets of Rocky Mountain Pipe ("Cor-Tek"), a manufacturer of PVC conduit for electrical applications, and considered a leading innovator in cellular core extrusion technology for a purchase price of $14,835 . In connection with this acquisition, the Company recorded a bargain purchase gain of $7,384 , which is reported within other income, net in the Consolidated Statements Of Operations . The Company believes that it was able to acquire the net assets of Cor-Tek for less than fair value as a result of Cor-Tek’s financial difficulties. On August 12, 2019, Unistrut Limited, a wholly-owned subsidiary of the Company, acquired Flytec Systems Ltd. and its parent holding company, Modern Associates Ltd., (collectively "Flytec"), a manufacturer of metal surface trunking, including IP4X, perimeter systems, pedestal boxes, as well as underfloor installations and industrial floor trunking. The purchase price was immaterial to the Company. On June 3, 2019, AFC Cable Systems, Inc., a wholly-owned subsidiary of the Company acquired the assets of United Structural Products, LLC. ("U.S. Tray"), a manufacturer of welded aluminum and engineered-to-order cable trays for a purchase price of $25,507 , net of cash received. The consolidated financial statements include the results of the acquired company from the acquisition date. Net sales and net income of the acquired company included in the consolidated statement of operations for fiscal 2019 were insignificant. As a result of the acquisition, the Company recognized $7,295 of goodwill, $14,800 of identifiable intangible assets and $3,412 of working capital and other net other tangible assets. On October 1, 2018, Allied Luxembourg S.a.r.l, a wholly-owned subsidiary of the Company acquired all of the outstanding stock of Vergokan International NV ("Vergokan") for a purchase price of $57,899 , net of cash received. Vergokan is a leading manufacturer of cable tray and cable ladder systems, underfloor installations and industrial floor trunking that serves industrial, power and energy, commercial and infrastructure sectors in more than 45 countries. This transaction provides Atkore with an expanded presence in Western Europe and strengthens the Company's electrical portfolio of cable management products within the Electrical Raceway segment. The Company incurred approximately $1,140 for acquisition related expenses for its 2019 acquisitions, which were recorded in selling, general, and administrative expenses. All the above acquisitions were funded with cash on hand. Fiscal 2018 On January 8, 2018, AFC Cable Systems, Inc., a wholly-owned subsidiary of the Company, acquired the assets of Communications Integrators, Inc. ("Cii"), a manufacturer of modular, prefabricated power, voice and data distribution systems located in Tempe, Arizona for a total purchase price, including contingent consideration, of $3,997 . The Cii acquisition complements Atkore's existing modular and prewired products within the Electrical Raceway segment. Purchase Price Allocations The purchase price was allocated to tangible and intangible assets acquired and liabilities assumed, based on their fair values. The following table summarizes the Level 3 fair values assigned to the net assets acquired and liabilities assumed as of the acquisition date for fiscal 2019: (in thousands) Vergokan Other Total Fair value of consideration transferred: Cash consideration $ 58,728 $ 41,641 $ 100,369 Other liability consideration — 1,400 1,400 Total consideration transferred 58,728 43,041 101,769 Fair value of assets acquired and liabilities assumed: Cash 829 1,541 2,370 Accounts receivable 8,761 8,217 16,978 Inventories 11,434 7,494 18,928 Intangible assets 12,621 16,400 29,021 Fixed assets 32,490 19,298 51,788 Accounts payable (18,716 ) (7,608 ) (26,324 ) Gain on purchase of business — (7,384 ) (7,384 ) Other 1,680 (3,412 ) (1,732 ) Net assets acquired 49,099 34,546 83,645 Excess purchase price attributed to goodwill acquired $ 9,629 $ 8,495 $ 18,124 Goodwill recognized from the Vergokan acquisition is non-deductible for tax purposes. Goodwill recognized from the other acquisitions is tax-deductible and is amortized over 15 years for income tax purposes. The goodwill consists largely of the synergies and economies of scale from integrating these companies with existing businesses. The following table summarizes the fair value of intangible assets as of the acquisition date: Vergokan Other ($ in thousands) Fair Value Weighted Average Useful Life (Years) Fair Value Weighted Average Useful Life (Years) Customer relationships $ 10,535 12.0 $ 15,400 10.0 Other 2,086 9.0 1,000 9.0 Total intangible assets $ 12,621 $ 16,400 Net sales and net income of the companies acquired during fiscal 2019 and 2018 are included in the consolidated statement of operations for fiscal 2019 and fiscal 2018 for the post-acquisition periods. Due to the immaterial nature of these acquisitions, both individually, and in the aggregate, the Company did not include the full year pro forma results of operations for the acquisition year or previous years.

DIVESTITURES

DIVESTITURES	12 Months Ended
	Sep. 30, 2020
Discontinued Operations and Disposal Groups [Abstract]
DIVESTITURES	4. DIVESTITURES On March 30, 2018, the Company sold the assets of FlexHead Industries, Inc. and SprinkFLEX, LLC (together "Flexhead"). The Flexhead businesses manufacture commercial flexible sprinkler head connection products for use in a variety of markets, including industrial, commercial, cold storage, institutional and clean room applications. The cash consideration received, net assets divested and resulting gain on sale for the year ended September 30, 2018 are as follows: (in thousands) Flexhead Cash consideration $ 42,631 Net assets divested 15,056 Gain on sale of a business $ 27,575 Net assets divested included $2,626 of goodwill.

POSTRETIREMENT BENEFITS

POSTRETIREMENT BENEFITS	12 Months Ended
	Sep. 30, 2020
Retirement Benefits [Abstract]
POSTRETIREMENT BENEFITS	5. POSTRETIREMENT BENEFITS The Company has a number of non-contributory and contributory defined benefit retirement plans covering certain United States employees. Net periodic pension benefit cost is based on periodic actuarial valuations that use the projected unit credit method of calculation and is charged to the statements of operations on a systematic basis over the expected average remaining service lives of current participants. The benefits under the defined benefit plans are based on various factors, such as years of service and compensation. For all periods presented, all defined pension benefit plans are frozen, whereby participants no longer accrue credited service. The net periodic (benefit)/cost for the periods presented was as follows: Fiscal Year Ended (in thousands) September 30, 2020 September 30, 2019 September 30, 2018 Interest cost 3,739 4,665 4,098 Expected return on plan assets (6,331 ) (6,372 ) (6,416 ) Amortization of actuarial loss 888 101 343 Net periodic (benefit) cost $ (1,704 ) $ (1,606 ) $ (1,975 ) The weighted-average assumptions used to determine net periodic pension (benefit) cost during the period were as follow: September 30, 2020 September 30, 2019 September 30, 2018 Discount rate 3.1 % 4.2 % 3.7 % Expected return on plan assets 6.3 % 6.3 % 6.4 % Rate of compensation increase N/a N/a N/a The change in the benefit obligations, plan assets and the amounts recognized on the consolidated balance sheets was as follows (in thousands): Change in benefit obligations: Balance as of September 30, 2018 $ 120,957 Interest cost 4,665 Actuarial loss 17,463 Benefits and administrative expenses paid (5,125 ) Balance as of September 30, 2019 137,960 Interest cost 3,739 Actuarial loss 10,476 Benefits and administrative expenses paid (5,416 ) Balance as of September 30, 2020 $ 146,759 Change in plan assets: Balance as of September 30, 2018 104,454 Actual return on plan assets 3,043 Employer contributions 1,079 Benefits and administrative expenses paid (5,125 ) Balance as of September 30, 2019 103,451 Actual return on plan assets 6,587 Employer contributions 2,114 Benefits and administrative expenses paid (5,416 ) Balance as of September 30, 2020 $ 106,736 Funded status: Funded status as of September 30, 2019 $ (34,509 ) Funded status as of September 30, 2020 $ (40,023 ) (in thousands) September 30, 2020 September 30, 2019 Amounts recognized in the consolidated balance sheets consist of: Pension liabilities $ (40,023 ) $ (34,509 ) Net amount recognized $ (40,023 ) $ (34,509 ) Amounts recognized in accumulated other comprehensive loss (before income taxes) consist of: Net actuarial loss $ (42,367 ) $ (33,035 ) Total loss recognized $ (42,367 ) $ (33,035 ) Weighted-average assumptions used to determine pension benefit obligations at year end: Discount rate 2.5 % 3.1 % Rate of compensation increase N/a N/a The Company elected to adopt the ASU 2018-14 early in the quarter ended September 30, 2020, which requires a narrative description of the reasons for significant gains and losses affecting the benefit obligation for the period. For the fiscal year ended 2020, the reduction in the discount rate from fiscal year 2019 to 2020 is the primary driver of the change in benefit obligations. The following table summarizes the defined benefit pension plans with accumulated benefit obligations in excess of plan assets: (in thousands) September 30, 2020 September 30, 2019 Accumulated benefit obligation $ 146,759 $ 137,960 Fair value of plan assets 106,736 103,451 The following table summarizes the defined benefit pension plans with projected benefit obligations in excess of plan assets: (in thousands) September 30, 2020 September 30, 2019 Projected benefit obligation $ 146,759 $ 137,960 Fair value of plan assets 106,736 103,451 In determining the expected return on plan assets, the Company considers the relative weighting of plan assets by class, historical performance of asset classes over long-term periods, asset class performance expectations as well as current and future economic conditions. The Company's investment strategy for its pension plans is to manage the plans on a going-concern basis. Current investment policy is to maximize the return on assets, subject to a prudent level of portfolio risk, for the purpose of enhancing the security of benefits for participants. For the pension plans, this policy targets a 60% allocation to equity securities and a 40% allocation to debt securities. Pension plans have the following weighted-average asset allocations: Asset Category: September 30, 2020 September 30, 2019 Equity securities 58% 58% Debt securities 39% 40% Cash and cash equivalents 3% 2% Total 100% 100% The Company evaluates its defined benefit plans' asset portfolios for the existence of significant concentrations of risk, such as investments in a single entity, industry, foreign country and individual fund manager. As of September 30, 2020 , there were no significant concentrations of risk in the Company's defined benefit plan assets. The Company's plan assets are accounted for at fair value and are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. The Company's assessment of the significance of a particular input to the fair value measurement requires judgment, and may affect the valuation of fair value of assets and their placement within the fair value hierarchy levels. The Company's asset allocations are presented in the table below: (in thousands) September 30, 2020 September 30, 2019 Level 1 Level 2 Total Level 1 Level 2 Total United States equity securities $ 30,738 $ 6,796 $ 37,534 $ 32,545 $ 6,429 $ 38,974 Non-U.S. equity securities 21,509 2,659 24,168 18,182 2,543 20,725 Fixed income securities 34,857 7,253 42,110 35,214 6,815 42,029 Cash and cash equivalents 2,924 — 2,924 1,723 — 1,723 Total $ 90,028 $ 16,708 $ 106,736 $ 87,664 $ 15,787 $ 103,451 Equity securities consist primarily of publicly traded United States and non-U.S. equities. Publicly traded securities are valued at the last trade or closing price reported in the active market in which the individual securities are traded. Certain equity securities are held within commingled funds, which are valued at the unitized net asset value ("NAV") or percentage of the NAV as determined by the custodian of the fund. These values are based on the fair value of the underlying net assets owned by the fund. Fixed income securities consist primarily of government and agency securities, corporate debt securities, and mortgage and other asset-backed securities. When available, fixed income securities are valued at the closing price reported in the active market in which the individual security is traded. Government and agency securities and corporate debt securities are valued using the most recent bid prices or occasionally the mean of the latest bid and ask prices when markets are less liquid. Asset-backed securities including mortgage-backed securities are valued using broker/dealer quotes when available. When quotes are not available, fair value is determined by utilizing a discounted cash flow approach, which incorporates other observable inputs such as cash flows, underlying security structure and market information including interest rates and bid evaluations of comparable securities. As of September 30, 2020 and September 30, 2019 , the Company did not have any Level 3 pension assets. Certain fixed income securities are held within commingled funds, which are valued utilizing NAV as determined by the custodian of the fund. These values are based on the fair value of the underlying net assets owned by the fund. Cash and cash equivalents consist primarily of short-term commercial paper, and other cash or cash-like instruments including settlement proceeds due from brokers, stated at cost, which approximates fair value. Transfers between levels of the fair value hierarchy are recognized on the actual date of the event or circumstance giving rise to the transfer, which generally coincides with the Company's valuation process. Contribution amounts are determined and funded based on laws and regulations and with the assistance of professionally qualified actuaries. The Company contributed $2,114 and $1,079 to its pension plans for the fiscal years ended September 30, 2020 and September 30, 2019 . The Company anticipates that it will contribute at least the minimum required contribution of $1,855 to its pension plans in fiscal 2021 . Benefit payments, which reflect future expected service as appropriate, are expected to be paid in each fiscal year as follows: (in thousands) 2021 $ 6,155 2022 6,436 2023 6,667 2024 6,879 2025 7,125 2026 to 2030 37,448 Defined Contribution Retirement Plans — The Company also sponsors several defined contribution retirement plans - the 401(k) matching programs. Expense for the defined contribution plans is computed as a percentage of participants' compensation and was $3,718 , $3,572 and $3,600 for the fiscal years ended September 30, 2020 , September 30, 2019 and September 30, 2018 , respectively. Multi-Employer Plan — The Company has a liability of $5,345 as of September 30, 2020 and $5,658 as of September 30, 2019 representing the Company's proportionate share of a multi-employer pension plan which was exited prior to fiscal 2017.

STOCK INCENTIVE PLAN

STOCK INCENTIVE PLAN	12 Months Ended
	Sep. 30, 2020
Share-based Payment Arrangement [Abstract]
STOCK INCENTIVE PLAN	6. STOCK INCENTIVE PLAN On November 21, 2019, the Company's board of directors approved the Atkore International Group Inc. 2020 Omnibus Incentive Plan (the "2020 Omnibus Incentive Plan"), which was subsequently approved by the Company’s shareholders on January 30, 2020. The 2020 Omnibus Incentive Plan provides for stock purchases and grants of other equity awards, including non-qualified stock options, stock purchase rights, restricted stock, restricted stock units ("RSUs"), performance shares, performance stock units ("PSUs"), stock appreciation rights, dividend equivalents and other stock-based awards to directors, officers, other employees and consultants. The 2020 Omnibus Incentive Plan replaces and succeeds the Atkore International Group Inc. 2016 Omnibus Incentive Plan (the "2016 Omnibus Incentive Plan"). The Company no longer grants awards from the 2016 Omnibus Incentive Plan. Awards previously granted under the 2016 Omnibus Incentive Plan were unaffected by the termination. A maximum of 1.1 million shares of common stock is reserved for issuance under the 2020 Omnibus Incentive Plan, plus the number of shares of common stock remaining available for issuance under the 2016 Omnibus Incentive Plan as of the date of the Company’s 2020 Annual Meeting of Stockholders. All awards have a ten year life. All share-based awards are expected to be fulfilled with new shares of common stock. Stock compensation expense is included in selling, general and administrative in the Company's consolidated statements of operations and was $13,064 , $11,798 and $14,664 for fiscal years 2020 , 2019 and 2018 , respectively. The total income tax benefit recognized for share-based compensation arrangements was $3,014 , $2,949 and $3,681 for fiscal years 2020 , 2019 and 2018 , respectively. On May 4, 2018, the Company's board of directors modified the stock awards of a key executive. As part of the modification, the Board extended the date through which the key executive has the right to exercise vested options. In addition, the amendment also accelerated the service vesting period of certain of his unvested stock options, RSUs and PSUs. As a result of these modifications, the Company recorded the entire incremental stock-based compensation expense of approximately $0.5 million during the year ended September 30, 2018. Stock Options In accordance with ASC 718 Compensation - Stock Compensation, stock compensation expense is recorded on a straight-line basis over the requisite service period (generally the vesting period), net of actual forfeitures based on the grant-date fair value of the option under the equity accounting method. The assumptions used in the Black-Scholes option pricing model to value the options granted were as follows: Fiscal Year Ended September 30, 2020 September 30, 2019 September 30, 2018 Expected dividend yield — % — % — % Expected volatility 36 % 45 % 40 % Range of risk-free interest rates 1.67 % 2.94% - 3.00% 2.16% - 2.38% Range of expected option lives 6 years 6 to 10 years 1 to 6 years Dividends are not paid on the Company's common stock. For grants during fiscal year ended 2020, the expected volatility is based on the Company's stock price volatility. For grants and modifications during fiscal years ended 2019 and 2020, expected volatility is based on historical volatilities of comparable companies. The risk-free interest rate is based on the United States Treasury yield curve in effect at the time of the grant for periods corresponding with the expected life of the options. The expected life of options is estimated using the simplified method due to limited historical exercise activity. The Company does not estimate forfeitures, which are accounted for as they occur. Stock option activity for the period September 30, 2017 to September 30, 2020 was as follows: Shares Weighted-Average Exercise Price Weighted-Average Grant Date Fair Value Aggregate Intrinsic Value (in thousands) Weighted-Average Remaining Contractual Term (in years) Outstanding as of September 30, 2017 5,158 $ 8.30 Granted 185 20.01 8.33 Exercised (2,728 ) 7.55 $ 42,512 Forfeited (10 ) 8.28 Outstanding as of September 30, 2018 2,605 9.91 Granted 342 23.45 11.35 Exercised (985 ) 8.62 $ 16,453 Forfeited (126 ) 12.64 Outstanding as of September 30, 2019 1,836 12.94 $ 31,957 Granted 67 37.24 $ 14.05 Exercised (508 ) 8.55 $ 14,548 Forfeited (20 ) 19.13 Outstanding as of September 30, 2020 1,375 15.66 11,443 5.43 Exercisable as of September 30, 2020 1,012 $ 11.99 $ 11,013 4.48 As of September 30, 2020 , there was $1,289 of total unrecognized compensation expense related to non-vested options granted expected to be recognized over a weighted-average period of approximately 1.2 years. The total fair value of shares vested during fiscal years 2020 , 2019 and 2018 was $1,894 , $3,038 and $9,757 , respectively. Cash received from stock option exercises for the fiscal years 2020 , 2019 and 2018 was $4,347 , $7,374 and $20,110 , respectively. The actual tax benefit for the tax deductions from stock option exercises totaled $3,681 , $4,031 and $11,478 , respectively, for fiscal years 2020 , 2019 and 2018 . The Company does not settle any option exercises, under its current stock incentive plan, in cash. Restricted Stock Units Generally, RSUs granted under the 2020 Omnibus Incentive Plan vest ratably over three years . The fair value of RSU grants was based on the closing price of the Company's common stock on the date of grant. RSU compensation expense is recorded on a straight-line basis over the remaining vesting period. Changes to the Company's nonvested RSU awards for the year ended September 30, 2020 were as follows: Shares Weighted-average grant-date fair value Nonvested as of September 30, 2017 435 $ 19.65 Granted 334 20.86 Vested (90 ) 21.95 Forfeited (29 ) 20.57 Nonvested as of September 30, 2018 650 $ 19.91 Granted 361 19.23 Vested (195 ) 21.51 Forfeited (142 ) 18.52 Nonvested as of September 30, 2019 674 19.37 Granted 166 37.24 Vested (403 ) 19.43 Forfeited (32 ) 22.29 Nonvested as of September 30, 2020 405 $ 26.44 As of September 30, 2020 , there was $5,185 of total unrecognized compensation expense related to non-vested RSUs granted, expected to be recognized over a weighted-average period of approximately 1.64 years. The total fair value of RSUs vested during fiscal years 2020 , 2019 and 2018 was $ 14,513 , $4,271 and $1,969 , respectively. Performance Share Units On November 27, 2017, November 29, 2018 and November 21, 2019 , the Company awarded PSUs whose vesting is contingent upon meeting or exceeding certain market and performance conditions. The performance condition, which was based on an adjusted net income, represented 70% of the award and the market condition, which was based on Total Shareholder Return ("TSR") of the Company's common stock relative to a peer group represented the remaining 30% . All PSUs cliff vest at the end of three years based on the satisfaction of the performance conditions. Expense for the performance condition based award is recorded when the achievement of the performance condition is considered probable of achievement and is recorded on a straight-line basis over the requisite service period. If such performance criteria are not met, no compensation cost is recognized and any recognized compensation cost is reversed. Expense for the market condition based award is recorded on a straight-line basis over the explicit service period. The grant-date fair value for the performance condition based awards represents the closing stock price on the date of grant. For the grants in fiscal 2020, 2019, and 2018, the closing stock price on the date of grant was $37.24 , $17.90 and $20.01 , respectively. The grant-date fair value for the market condition based awards was determined using the Monte-Carlo method. The assumptions used in the Monte-Carlo method to value the performance share awards granted during the fiscal year ended September 30, 2020 were as follows: September 30, 2020 September 30, 2019 September 30, 2018 Expected dividend yield — % — % — % Range of expected volatility 21.45-53.18 20.69 - 74.26 17.21 - 79.15 Risk free interest rates 1.59 % 2.84 % 1.83% - 2.54% Expected life 3 years 3 years 1 -3 years Fair value $ 44.22 18.84 $ 13.90 - 26.68 Dividends are not paid on the Company's common stock. Expected volatility is based on historical volatilities of comparable companies. The risk-free interest rate is based on the United States Treasury yield curve in effect at the time of the grant for periods corresponding with the expected life of the award. The expected life of the award represents the weighted-average period of time that awards granted are expected to be outstanding, giving consideration to vesting schedules and expected exercise patterns. The Company does not estimate forfeitures, which are accounted for as they occur. Changes to the Company's non-vested PSU awards for the year ended September 30, 2020 were as follows: Shares Weighted-average grant-date fair value Nonvested as of September 30, 2017 181 $ 23.87 Granted 215 22.01 Forfeited (89 ) 28.53 Nonvested as of September 30, 2018 307 $ 21.22 Granted 242 18.17 Forfeited (94 ) 20.55 Nonvested as of September 30, 2019 455 $ 21.11 Granted 115 40.43 Vested (162 ) 25.21 Adjustment for achieved performance upon issuance 31 21.68 Forfeited (14 ) 22.82 Nonvested as of September 30, 2020 425 $ 25.15 As of September 30, 2020 , there was $2,783 of total unrecognized compensation expense related to non-vested PSUs granted, expected to be recognized over a weighted-average period of approximately 1.60 years. There were no PSUs vested during fiscal years 2019 and 2018 .

RESTRUCTURING CHARGES

RESTRUCTURING CHARGES	12 Months Ended
	Sep. 30, 2020
Restructuring and Related Activities [Abstract]
Restructuring Charges	7. RESTRUCTURING CHARGES Restructuring charges relate to the streamlining of the Company's cost structure and improving its operations. These actions primarily resulted in workforce reductions, lease termination costs, and other facility rationalization costs. Restructuring reserves are included as a component of other current liabilities . Electrical Raceway Mechanical Products & Solutions Other/Corporate (in thousands) Severance (a) Other (b) Severance (a) Other (a) Severance (a) Total Balance as of September 30, 2017 $ 449 $ — $ 278 $ 10 $ — $ 737 Charges 536 1,130 97 179 98 2,040 Utilization (787 ) (820 ) (178 ) (160 ) (98 ) (2,043 ) Reversals — — (191 ) — — (191 ) Exchange rate effects 14 — (6 ) — — 8 Balance as of September 30, 2018 $ 212 $ 310 $ — $ 29 $ — $ 551 Charges 1,047 2,544 213 — — 3,804 Utilization (867 ) (2,854 ) (68 ) (29 ) — (3,818 ) Balance as of September 30, 2019 $ 392 $ — $ 145 $ — $ — $ 537 Charges $ 1,339 $ 1,892 $ 416 — 68 3,715 Utilization $ (1,175 ) $ (1,892 ) $ (429 ) — (52 ) (3,548 ) Reversals $ (369 ) $ — $ (60 ) — (2 ) (431 ) Exchange rate effects $ 18 $ — $ — — — 18 Balance as of September 30, 2020 $ 205 $ — $ 72 $ — $ 14 $ 291 (a) Fiscal 2020 charges are primarily related to a reduction in force implemented as part of adjusting to the macroeconomic impacts related to the outbreak of the novel coronavirus (COVID-19). Fiscal 2019 and 2018 charges are primarily related to Atkore's commitment to close and consolidate certain facilities as part of its continuing effort to realign its strategic focus. (b) Primarily related to Atkore's commitment to close and consolidate certain facilities as part of its continuing effort to realign its strategic focus. The Company expects to utilize all restructuring accruals as of September 30, 2020 within the next twelve months. The net restructuring charges included as a component of selling, general and administrative expense in the Company's consolidated statements of operations were as follows: Fiscal Year Ended (in thousands) September 30, 2020 September 30, 2019 September 30, 2018 Total restructuring charges, net $ 3,284 $ 3,804 $ 1,849

OTHER INCOME, NET

OTHER INCOME, NET	12 Months Ended
	Sep. 30, 2020
Other Income, Nonoperating [Abstract]
OTHER INCOME, NET	8. OTHER INCOME, NET Other income, net consisted of the following: Fiscal Year Ended (in thousands) September 30, 2020 September 30, 2019 September 30, 2018 Gain on purchase of business $ — $ (7,384 ) $ — Gain on sale of a business — — (27,575 ) Undesignated foreign currency derivative instruments 1,269 (5,384 ) (121 ) Foreign exchange loss (gain) on intercompany loans (2,342 ) 2,975 1,500 Debt modification costs — — 892 Pension-related benefits (1,704 ) (1,606 ) (1,975 ) Other — (79 ) (69 ) Other income, net $ (2,777 ) $ (11,478 ) $ (27,348 )

INCOME TAXES

INCOME TAXES	12 Months Ended
	Sep. 30, 2020
Income Tax Disclosure [Abstract]
INCOME TAXES	9. INCOME TAXES On March 27, 2020, President Trump signed into U.S. federal law the Coronavirus Aid, Relief and Economic Security ("CARES") Act, which is aimed at providing emergency assistance and health care for individuals, families, and businesses affected by the COVID-19 pandemic and generally supporting the U.S. economy. The CARES Act, among other things, includes provisions related to refundable payroll tax credits, deferment of the employer portion of social security payments, net operating loss carry back periods, modifications to the net interest deduction limitations, and technical corrections to tax depreciation methods for qualified improvement property. There is no material impact to our consolidated financial statements as a result of this legislation. On December 22, 2017, "H.R.1," also known as the Tax Cuts and Jobs Act ("TCJA") was signed into law. This legislation provided for significant changes to corporate taxation including, but not limited to, a reduction of the federal corporate tax rate from 35% to 21% , limitations on the deductibility of interest expense and executive compensation, full expensing of the costs of qualified property in the period of acquisition and the elimination of the domestic production activities deduction. The legislation also adopts a new quasi-territorial tax regime and imposes a one-time transition tax on deemed repatriated earnings of certain foreign subsidiaries. As part of the enactment of TCJA, the Company recorded a global intangible low-taxed income ("GILTI") provision for the first time beginning in fiscal 2019. The GILTI provision of TCJA requires certain income earned by controlled foreign corporations ("CFCs") to be included currently in the gross income of the CFCs controlling U.S. shareholder. In accordance with accounting standards applicable to income taxes, there is allowed an accounting policy choice of either (1) treating taxes due on U.S. inclusions in taxable income related to GILTI as a current period expense when incurred (the “period cost method”) or (2) factoring such amounts into the Company’s measurement of its deferred taxes (the “deferred method”). The Company has elected the period method. Significant components of income before income taxes and income tax expense for the fiscal years ended September 30, 2020 , September 30, 2019 and September 30, 2018 consisted of the following: (in thousands) September 30, 2020 September 30, 2019 September 30, 2018 Components of income before income taxes: United States $ 201,784 $ 170,780 $ 155,453 Non-U.S 214 13,889 10,899 Income before income taxes $ 201,998 $ 184,669 $ 166,352 Income tax expense: Current: United States: Federal $ 32,335 $ 31,431 $ 26,586 State 9,668 9,800 8,662 Non-U.S: 3,210 5,183 3,467 Current income tax expense $ 45,213 $ 46,414 $ 38,715 Deferred: United States: Federal $ 5,102 $ 1,123 $ (7,149 ) State 49 (340 ) (1,793 ) Non-U.S: (668 ) (1,579 ) (66 ) Deferred income (benefit) tax expense 4,483 (796 ) (9,008 ) Income tax expense $ 49,696 $ 45,618 $ 29,707 In addition to the change in the U.S. federal statutory rate, the mix of foreign and domestic losses, along with rate reconciling items as outlined below, impact the Company's effective tax rate. Differences between the statutory federal income tax rate and effective income tax rate are summarized below: (in thousands) September 30, 2020 September 30, 2019 September 30, 2018 Statutory federal tax 21 % 21 % 25 % Adjustments to reconcile to the effective income tax rate: State income taxes 4 % 4 % 3 % Change in tax rates — % — % (3 )% Domestic manufacturing deduction — % — % (2 )% Indemnified uncertain tax benefits — % — % (2 )% Stock-based compensation (2 )% (1 )% (3 )% Other 2 % 1 % — % Effective income tax rate 25 % 25 % 18 % The Company's effective tax rate for fiscal 2020 differs from the statutory rate primarily due to state income taxes of $7,690 , current year valuation allowance expense of $2,099 and limitations on executive compensation of $1,243 partially offset by $4,203 of excess tax benefit associated with the exercise of stock options. The Company's effective tax rate for fiscal 2019 differs from the statutory rate primarily due to state income taxes of $ 7,566 , along with GILTI provisions of $ 953 and withholding taxes of $ 635 , offset by the disallowed gain on the purchase of Cor-Tek of $ 1,551 and $ 1,537 of excess tax benefit associated with the exercise of stock options. The Company's effective tax rate for fiscal 2018 differs from the statutory rate due to a $5,466 benefit from the remeasurement of the deferred tax liabilities as a result of the TCJA, a $4,091 excess tax benefit associated with the exercise of stock options, a net tax benefit of $3,977 from the release of indemnified tax positions and a benefit of $3,309 for the domestic manufacturing deduction, partially offset by $5,675 of state income tax expense. Deferred income taxes result from temporary differences between the amount of assets and liabilities recognized for financial reporting and tax purposes. The components of the net deferred income tax assets are as follows: (in thousands) September 30, 2020 September 30, 2019 Deferred tax assets: Accrued liabilities and reserves $ 26,783 $ 32,664 Tax loss and credit carryforwards 15,637 13,296 Postretirement benefits 11,751 10,315 Inventory 5,240 8,813 Lease obligations 9,966 — Other 174 916 $ 69,551 $ 66,004 Deferred tax liabilities: Property, plant and equipment $ (17,648 ) $ (21,312 ) Intangible assets (44,774 ) (47,887 ) Loss on investment (5,153 ) (4,790 ) Right-of-use assets, net (9,546 ) — Other (4,065 ) (3,660 ) $ (81,186 ) $ (77,649 ) Net deferred tax liability before valuation allowance (11,635 ) (11,645 ) Valuation allowance (10,203 ) (7,764 ) Net deferred tax liability $ (21,838 ) $ (19,409 ) As of September 30, 2020 , the Company has $44,169 of state net operating loss carryforwards which expire beginning in 2020 through 2035. In certain non-U.S. jurisdictions, the Company has net operating loss carryforwards of $52,092 which have an expiration period ranging from five years to unlimited. Valuation allowances have been established on net operating losses and other deferred tax assets in Australia, France, Asia Pacific, and other foreign and United States state jurisdictions, as a result of the Company's determination that there is less than 50% likelihood that these assets will be realized. Evidence for this determination includes three year cumulative loss positions, future reversal of temporary differences, and expectations of future losses. For fiscal 2019, the Company assessed the need for a continued valuation allowance against deferred tax assets in the Company's Asia Pacific business based on recent taxable income and the expected future performance of the business. As a result, the Company has released a $550 valuation allowance against the deferred tax assets of the Asia Pacific business. As of September 30, 2020 , the Company had unrecognized tax benefits of $1,614 which, if recognized, would positively benefit the effective tax rate. The Company recognizes interest and penalties related to unrecognized tax benefits in income tax expense. As of September 30, 2020 and September 30, 2019 , the Company had accrued interest and penalties of $188 and $54 , respectively, in the consolidated balance sheets. A reconciliation of the beginning and ending amount of unrecognized tax benefit, excluding interest and penalties, is as follows: (in thousands) For the period from September 30, 2017 to September 30, 2020 Balance as of September 30, 2017 $ 3,578 Additions based on tax positions related to prior years 893 Settlements (3,107 ) Balance as of September 30, 2018 1,364 Additions based on tax positions related to prior years 244 Settlements (814 ) Balance as of September 30, 2019 794 Additions based on tax positions related to prior years 925 Additions based on tax positions related to current year 122 Settlements (227 ) Balance as of September 30, 2020 1,614 During fiscal 2020, the balance of unrecognized tax benefits increased by $1,047 as a result of federal and various state jurisdictions' uncertain tax positions, partially offset by a decrease of $227 as result of completing tax audits and the expiration of the statute of limitations in various state jurisdictions. The related accrued penalties and interest for uncertain tax positions increased by $134 . During fiscal 2019 , the balance of unrecognized tax benefits decreased by $814 as a result of completing state tax audits and the expiration of the statute of limitations in federal and various state jurisdictions, partially offset by an increase of $244 , primarily related to various state jurisdictions' uncertain tax positions. The related accrued penalties and interest for uncertain tax positions decreased by $25 . During fiscal 2018 , the balance of unrecognized tax benefits decreased by $3,107 as a result of Tyco completing tax audits and the expiration of the statute of limitations in various state jurisdictions, partially offset by an increase of $893 , primarily related to various state jurisdictions' uncertain tax positions. The related accrued penalties and interest for uncertain tax positions decreased by $3,139 . Many of the Company's uncertain tax positions relate to tax years that remain subject to audit by the taxing authorities. The following tax years remain subject to examination by the major tax jurisdictions as follows: Jurisdiction Years Open to Audit United States 2018, 2019 and 2020 The Company's income tax returns are examined periodically by various taxing authorities. The Company is currently under examination in various state jurisdictions. Based on the current status of its income tax audits, the Company believes that it is reasonably possible that there would be no material changes to the unrecognized tax benefits in the next twelve months. Other Income Tax Matters —Prior to the passage of the TCJA, foreign undistributed earnings were generally subject to U.S. taxation when repatriated. The TCJA imposed a one-time transition tax on previously untaxed accumulated earnings of foreign subsidiaries. The Company has accumulated earnings and profits deficit, therefore, does not anticipate an additional tax liability as a result of the transition tax. The TCJA adopts a new quasi-territorial tax regime that eliminates U.S income taxes on dividends from foreign subsidiaries. The Company may still be liable for foreign taxes, such as withholding taxes, if earnings are repatriated. For the fiscal year ended September 30, 2020 , the Company recorded no additional liability for United States or non-U.S. income taxes on the undistributed income of subsidiaries or for unrecognized deferred tax liabilities for temporary differences related to basis differences in investments in subsidiaries, as such income is expected to be indefinitely reinvested, the investments are essentially permanent in duration, or the Company has concluded that no additional tax liability will arise as a result of the distribution of such income. For the fiscal year ended September 30, 2019 , the Company recorded a liability of $635 , for withholding taxes on the planned distribution of income from one of its Europe subsidiaries. The Company did not record income tax or non-income tax expense related to the remaining foreign earnings, and did not record any deferred tax liabilities for any basis differences in investments in subsidiaries as the earnings are expected to be indefinitely reinvested, the investments are essentially permanent in duration, or the Company has concluded that there will be no additional tax liability as a result of the distribution of the income. As of September 30, 2020 , certain subsidiaries had approximately $59,719 of undistributed income that the Company intends to permanently reinvest. A liability could arise if the Company's intention to permanently reinvest such income were to change and amounts are distributed by such subsidiaries or if such subsidiaries are ultimately disposed of. It is not practicable to estimate the additional income taxes related to permanently reinvested income or the basis differences related to investments in subsidiaries. The calculation of the Company's tax liabilities involves dealing with uncertainties in the application of complex tax regulations in a multitude of jurisdictions across its global operations. The Company records tax liabilities for anticipated tax audit issues in the United States and other tax jurisdictions based on the Company's estimate of whether, and the extent to which, additional taxes will be due. These tax liabilities are reflected net of related tax loss carry-forwards. The Company adjusts these reserves in light of changing facts and circumstances. However, due to the complexity of some of these uncertainties, the ultimate resolution may result in a payment that is materially different from the Company's current estimate of the tax liabilities. Under the terms of an investment agreement between the Company and Tyco, Tyco has agreed to indemnify and hold harmless the Company and its subsidiaries and their respective affiliates from and against any taxes of the Company with respect to any tax period ending on or before the closing of the Transactions, as well as all tax liabilities relating to events or transactions occurring on or prior to the closing date of the Transactions. In addition, the Company has agreed to indemnify and hold harmless Tyco and its affiliates from and against any liability for any taxes of the Company with respect to any post-Transactions tax period.

EARNINGS PER SHARE

EARNINGS PER SHARE	12 Months Ended
	Sep. 30, 2020
Earnings Per Share [Abstract]
EARNINGS PER SHARE	10. EARNINGS PER SHARE The Company calculates basic and diluted earnings per common share using the two-class method. Under the two-class method, net earnings are allocated to each class of common stock and participating securities as if all of the net earnings for the period had been distributed. The Company's participating securities consist of share-based payment awards that contain a non-forfeitable right to receive dividends and therefore are considered to participate in undistributed earnings with common stockholders. Basic earnings per common share excludes dilution and is calculated by dividing the net earnings allocable to common stock by the weighted-average number of common stock outstanding for the period. Diluted earnings per common share is calculated by dividing net earnings allocated to common stock by the weighted-average number of shares outstanding for the period, as adjusted for the potential dilutive effect of non-participating share-based awards. The following table sets forth the computation of basic and diluted earnings per share: Fiscal Year Ended (in thousands, except per share data) September 30, 2020 September 30, 2019 September 30, 2018 Numerator: Net income $ 152,302 $ 139,051 $ 136,645 Less: Undistributed earnings allocated to participating securities 3,356 3,726 2,456 Net income available to common shareholders $ 148,946 $ 135,325 $ 134,189 Denominator: Basic weighted average common shares outstanding 47,265 46,577 51,791 Effect of dilutive securities: Non-participating employee stock options (1) 779 1,200 2,298 Diluted weighted average common shares outstanding 48,044 47,777 54,089 Basic earnings per share $ 3.15 $ 2.91 $ 2.59 Diluted earnings per share $ 3.10 $ 2.83 $ 2.48 (1) Stock options to purchase approximately 0.3 million , 0.4 million , and 0.3 million shares of common stock were outstanding during the years ended September 30, 2020 , September 30, 2019 , and September 30, 2018 , respectively, but were not included in the calculation of diluted earnings per share as the impact of these would have been anti-dilutive.

ACCUMULATED OTHER COMPREHENSIVE

ACCUMULATED OTHER COMPREHENSIVE LOSS	12 Months Ended
	Sep. 30, 2020
Equity [Abstract]
ACCUMULATED OTHER COMPREHENSIVE LOSS	11. ACCUMULATED OTHER COMPREHENSIVE LOSS The following table presents the changes in accumulated other comprehensive loss by component, net of tax: (in thousands) Defined benefit pension items Currency translation adjustments Total Balance as of September 30, 2018 $ (6,048 ) $ (10,390 ) $ (16,438 ) Other comprehensive loss before reclassifications (15,513 ) (7,490 ) (23,003 ) Amounts reclassified from accumulated other comprehensive loss 76 — 76 Net current period other comprehensive loss (15,437 ) (7,490 ) (22,927 ) Reclassification of stranded tax benefits (1) (2,333 ) — (2,333 ) Balance as of September 30, 2019 $ (23,818 ) $ (17,880 ) (41,698 ) Other comprehensive income (loss) before reclassifications (7,604 ) 6,087 (1,517 ) Amounts reclassified from accumulated other comprehensive loss 661 — 661 Net current period other comprehensive income (loss) (6,943 ) 6,087 (856 ) Balance as of September 30, 2020 $ (30,761 ) $ (11,793 ) $ (42,554 ) (1) Due to the adoption of ASU 2018-02 stranded tax benefits related to its pension plans were reclassified from Accumulated other comprehensive loss and into Accumulated deficit The following is a summary of the amounts reclassified from accumulated other comprehensive loss to net income : Fiscal Year Ended (in thousands) September 30, 2020 September 30, 2019 September 30, 2018 Amortization of defined benefit pension items: Amortization of net loss (included within other income, net) $ 888 $ 101 $ 343 Tax expense (227 ) (25 ) (14 ) Net reclassifications for the period $ 661 $ 76 $ 329

INVENTORIES, NET

INVENTORIES, NET	12 Months Ended
	Sep. 30, 2020
Inventory Disclosure [Abstract]
INVENTORIES, NET	12. INVENTORIES, NET A majority of the Company records inventory at the lower of cost (primarily last in, first out, or "LIFO") or market or net realizable value, as applicable. Approximately 73% and 72% of the Company's inventories are valued at the lower of LIFO cost or market at September 30, 2020 and September 30, 2019 , respectively. (in thousands) September 30, 2020 September 30, 2019 Purchased materials and manufactured parts, net $ 49,192 $ 52,742 Work in process, net 24,113 21,424 Finished goods, net 125,790 151,924 Inventories, net $ 199,095 $ 226,090 Total inventories would be $4,418 lower and $3,138 higher than reported as of September 30, 2020 and September 30, 2019 , respectively, if the first-in, first-out method was used for all inventories. During the year ended September 30, 2020, inventory quantities were reduced. This reduction resulted in a liquidation of LIFO inventory quantities carried at net lower costs prevailing in prior years as compared with the cost of 2020 purchases, the effect of which decreased cost of goods sold and increased operating income by approximately $1,837 . As of September 30, 2020 and September 30, 2019 , the excess and obsolete inventory reserve was $14,533 and $14,295

PROPERTY, PLANT AND EQUIPMENT

PROPERTY, PLANT AND EQUIPMENT	12 Months Ended
	Sep. 30, 2020
Property, Plant and Equipment [Abstract]
PROPERTY, PLANT AND EQUIPMENT	13. PROPERTY, PLANT AND EQUIPMENT As of September 30, 2020 and September 30, 2019 , property, plant and equipment at cost and accumulated depreciation were as follows: (in thousands) September 30, 2020 September 30, 2019 Land $ 20,460 $ 19,897 Buildings and related improvements 129,538 127,061 Machinery and equipment 332,260 318,421 Leasehold improvements 9,862 9,055 Software 27,028 24,835 Construction in progress 22,736 21,264 Property, plant and equipment 541,884 520,533 Accumulated depreciation (297,993 ) (259,830 ) Property, plant and equipment, net $ 243,891 $ 260,703 Depreciation expense for fiscal years ended September 30, 2020 , September 30, 2019 and September 30, 2018 totaled $42,208 , $39,471 and $34,786

GOODWILL AND INTANGIBLE ASSETS

GOODWILL AND INTANGIBLE ASSETS	12 Months Ended
	Sep. 30, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
GOODWILL AND INTANGIBLE ASSETS	14. GOODWILL AND INTANGIBLE ASSETS Goodwill — C hanges in the carrying amount of goodwill are as follows: Segment (in thousands) Electrical Raceway Mechanical Products & Solutions Total Balance as of September 30, 2018 $ 133,566 $ 36,563 $ 170,129 Goodwill acquired during year 18,124 — 18,124 Exchange rate effects $ (2,022 ) — $ (2,022 ) Balance as of September 30, 2019 $ 149,668 $ 36,563 $ 186,231 Exchange rate effects 2,008 — 2,008 Balance as of September 30, 2020 $ 151,676 $ 36,563 $ 188,239 Goodwill balances include $3,924 and $43,000 of accumulated impairment losses within the Electrical Raceway and MP&S segments, respectively, as of September 30, 2020 and September 30, 2019 . Intangible Assets — The following table provides the gross carrying value, accumulated amortization, and net carrying value for each major class of intangible assets: September 30, 2020 September 30, 2019 (in thousands) Weighted Average Useful Life (Years) Gross Carrying Value Accumulated Amortization Net Carrying Value Gross Carrying Value Accumulated Amortization Net Carrying Value Amortizable Intangible Assets: Customer relationships 11 $ 355,735 $ (202,677 ) $ 153,058 $ 353,256 $ (171,777 ) $ 181,479 Other 7 19,086 (9,675 ) 9,411 19,086 (7,761 ) 11,325 Total 374,821 (212,352 ) 162,469 372,342 (179,538 ) 192,804 Indefinite-lived Intangible Assets: Trade names 92,880 — 92,880 92,880 — 92,880 Total $ 467,701 $ (212,352 ) $ 255,349 $ 465,222 $ (179,538 ) $ 285,684 Amortization expense for the fiscal years ended September 30, 2020 , September 30, 2019 and September 30, 2018 was $32,262 , $32,876 and $32,104 , respectively. Expected amortization expense for intangible assets over the next five years and thereafter is as follows (in thousands) : 2021 $ 32,267 2022 30,859 2023 30,806 2024 26,287 2025 13,681 2026 and thereafter 28,569

DEBT

DEBT	12 Months Ended
	Sep. 30, 2020
Debt Disclosure [Abstract]
DEBT	15. DEBT Debt as of September 30, 2020 and September 30, 2019 was as follows: (in thousands) September 30, 2020 September 30, 2019 First Lien Term Loan Facility due December 22, 2023 811,540 $ 851,361 Deferred financing costs (7,804 ) (6,569 ) Other — 525 Total debt $ 803,736 $ 845,317 Less: Current portion — — Long-term debt $ 803,736 $ 845,317 During the three months ended March 29, 2019, the Company made an accelerated repayment of $ 18,680 of principal on the First Lien Loan, which was calculated by a formula based on 2018 excess cash flows and a leverage ratio as defined within the Term Loan Agreement. During the three months ended September 30, 2019, the Company made a voluntary prepayment of $40,000 of principal on the First Lien Loan. During the three months ended September 30, 2020, the Company made a voluntary prepayment of $40,000 of principal on the First Lien Loan. The voluntary prepayments in the past two years resulted in the removal of all principal payment requirements until the contractual maturity of the debt in fiscal 2024. As of September 30, 2020 , future contractual maturities of long-term debt are as follows (in thousands) : 2021 $ — 2022 — 2023 — 2024 812,120 2025 — 2026 and thereafter $ — Term Loan Facility — On April 9, 2014, AII entered into a credit agreement (the "Initial Credit Agreement") for a $420,000 First Lien Term Loan Facility (the "Initial First Lien Term Loan Facility") and a credit agreement for a $ 250,000 Second Lien Term Loan Facility (the "Second Lien Term Loan Facility." The Initial First Lien Term Loan Facility was priced at 99.50% and carried an interest rate of LIBOR plus 3.50% with a LIBOR floor of 1.00% . The Second Lien Term Loan Facility was priced at 99.00% and carried an interest rate of LIBOR plus 6.75% with a LIBOR floor of 1.00% . On December 22, 2016, AII entered into an amendment to the Initial Credit Agreement, which amended and restated the Initial Credit Agreement and provided for a new $500,000 first lien term loan facility and extended the maturity of the Initial Credit Agreement to December 22, 2023 (the "First Lien Term Loan Facility"). Loans under the First Lien Term Loan Facility bear interest at either LIBOR plus an applicable margin equal to 3.00% or an alternate base rate plus an applicable margin equal to 2.00% and are guaranteed by AIH and the United States operating companies owned by AII. The First Lien Term Loan Facility amortizes at a rate of 1.00% per annum and was priced at 99.75% . AII used proceeds from the First Lien Term Loan Facility and approximately $155,000 of available cash to (i) repay all outstanding loans under the Initial First Lien Term Loan Facility and the Second Lien Term Loan Facility and (ii) pay related fees and expenses, including accrued interest. On February 2, 2018 , AII entered into the (i) First Amendment to the Amended and Restated First Lien Credit Agreement, by and among AII, Deutsche Bank AG New York Branch, as administrative agent and collateral agent, and the other financial institutions party thereto to, among other things, decrease the interest margins applicable to the ABR Loans to 1.75% and Eurodollar Loans to 2.75% , and (ii) Increase Supplement (the "Increase Supplement") to, among other things, incur incremental first lien secured term loans in aggregate principal amount of $425,000 . The Company used the proceeds from the Increase Supplement to 1) repurchase approximately 17.2 million shares of common stock from the CD&R Investor for a total purchase price of approximately $375,000 , 2) repay $42,000 of outstanding loans under the ABL Credit Facility (as defined below) and 3) pay $5,800 in related fees and expenses. The revisions to the First Lien Term Loan were accounted for as a debt modification, resulting in immediate expensing of related financing costs of $892 within other income, net on the consolidated statements of operations for the year ended September 30, 2018. The First Lien Term Loan Facility contains customary covenants typical for this type of financing, including limitations on indebtedness, restricted payments including dividends, liens, restrictions on distributions from restricted subsidiaries, sales of assets, affiliate transactions, mergers and consolidations. The First Lien Term Loan Facility also contains customary events of default typical for this type of financing, including, without limitation, failure to pay principal and/or interest when due, failure to observe covenants, certain events of bankruptcy, the rendering of certain judgments, or the loss of any guarantee. ABL Credit Facility — On December 22, 2016, AII entered into the Fifth Amendment to Credit Agreement and Third Amendment to and Reaffirmation of Guarantee and Collateral Agreement to amend its asset based credit facility (the "ABL Credit Facility"). The amendment, among other things, extended the maturity of the facility to December 22, 2021, decreased the interest rate margins applicable to loans under the facility to (i) in the case of United States dollar-denominated loans, either (x) LIBOR plus an applicable margin ranging from 1.25% to 1.75% , or (y) an alternate base rate plus an applicable margin ranging from 0.25% to 0.75% or (ii) in the case of Canadian dollar-denominated loans, either (x) the BA rate plus an applicable margin ranging from 1.25% to 1.75% or (y) a Canadian prime rate plus an applicable margin ranging from 0.25% to 0.75% and decreased the fee payable with respect to unutilized availability under the facility from 0.25% to 0.375% depending on the remaining availability under the facility. On August 28 2020, AII amended the ABL Credit Facility (the "Amended ABL Facility"). The amendment, among other things, extended the maturity of the facility to August 28, 2023 , and increased the interest rate margins applicable to loans under the facility to (i) in the case of United States dollar-denominated loans, either (x) LIBOR plus an applicable margin ranging from 1.75% to 2.25% , or (y) an alternate base rate plus an applicable margin ranging from 0.75% to 1.25% , each based on available loan commitments or (ii) in the case of Canadian dollar-denominated loans, either (x) the BA rate plus an applicable margin ranging from 1.75% to 2.25% or (y) a Canadian prime rate plus an applicable margin ranging from 0.75% to 1.25% , each based on available loan commitments. The Amended ABL Credit Facility bears a commitment fee, payable quarterly in arrears, of 0.375% per annum. The Amended ABL Credit Facility also bears customary letter of credit fees. The revisions to the ABL Credit Facility were accounted for as a debt extinguishment, resulting in immediate expensing of unamortized financing costs of $273 for the year ended September 30, 2020. In connection with AII causing Columbia-MBF to become a subsidiary guarantor for the Amended ABL Credit Facility, AII also caused Columbia-MBF to become a subsidiary guarantor for the First Lien Term Loan Facility. The Amended ABL Credit Facility has aggregate commitments of $325,000 and is guaranteed by AIH, the United States subsidiaries owned directly or indirectly by AII and certain other restricted subsidiaries of AII that AII causes to be a subsidiary guarantor from time to time including as of the closing date for the Amended ABL Credit Facility, Columbia-MBF, Inc., a corporation formed by amalgamation under the laws of Canada ("Columbia-MBF"). AII's availability under the ABL Credit Facility was $265,899 and $301,882 as of September 30, 2020 and September 30, 2019 , respectively. Availability under the ABL Credit Facility is subject to a borrowing base equal to the sum of 85% of eligible accounts receivable plus the lesser of (i) 80% of eligible inventory of each borrower and guarantor, valued at the lower of cost and fairmarket value and (ii) 85% of the net orderly liquidation value of eligible inventory, subject to certain limitations. There were no borrowings outstanding under the ABL Credit Facility as of September 30, 2020 and September 30, 2019 , respectively. The Amended ABL Credit Facility contains customary representations and warranties and customary affirmative and negative covenants. Affirmative covenants include, without limitation, the timely delivery of quarterly and annual financial statements, certifications to be made by AIH, AII and each of its restricted subsidiaries, payment of obligations, maintenance of corporate existence and insurance, notices, compliance with environmental laws, and the grant of liens. The negative covenants are limited to the following: limitations on indebtedness, dividends and distributions, investments, acquisitions, prepayments or redemptions of subordinated indebtedness, amendments of subordinated indebtedness, transactions with affiliates, asset sales, mergers, consolidations and sales of all or substantially all assets, liens, negative pledge clauses, changes in line of business, changes in charter documents, hedging transactions and maintaining Canadian defined benefit pension plans. The negative covenants are subject to customary exceptions and also permit the payment of dividends and distributions, investments, permitted acquisitions, payments or redemptions of subordinated indebtedness, asset sales and consolidations and sales of all or substantially all assets upon satisfaction of a payment condition. There are no financial covenants included in the Amended ABL Credit Agreement, other than a springing minimum fixed charge coverage ratio of at least 1.0 to 1.0, which will be tested only when specified availability (as defined above) is less than the greater of (A) $22,000 and (B) 10.0% of the lesser of (x) the then applicable borrowing base and (y) the then effective commitments under the Amended ABL Credit Facility, and continuing until such time as specified availability shall have been in excess of such threshold for a period of 30 consecutive days.

FAIR VALUE MEASUREMENTS

FAIR VALUE MEASUREMENTS	12 Months Ended
	Sep. 30, 2020
Fair Value Disclosures [Abstract]
FAIR VALUE MEASUREMENTS	16. FAIR VALUE MEASUREMENTS Certain assets and liabilities are required to be recorded at fair value on a recurring basis. The Company uses forward currency contracts to hedge the effects of foreign exchange relating to certain of the Company's intercompany receivables denominated in a foreign currency. These derivative instruments are not formally designated as hedges by the Company and the terms of these instruments range from six months to two years . Short-term forward currency contracts are recorded in prepaid expenses and other current assets or other current liabilities and long-term forward currency contracts are recorded in non-trade receivables or other long-term liabilities in the consolidated balance sheets . The fair value gains and losses are included in other income, net within the consolidated statements of operations . See Note 8, ''Other Income, net'' for further detail. The total notional amounts of undesignated forward currency contracts were £ 43.3 million, £ 45.0 million and £ 49.1 million as of September 30, 2020 , September 30, 2019 and September 30, 2018 . Cash flows associated with derivative financial instruments are recognized in the operating section of the consolidated statements of cash flows . The fair value of forward currency contracts is calculated by reference to current forward exchange rates for contracts with similar maturity profiles. The following table presents the recurring assets and liabilities measured at fair value as of September 30, 2020 and September 30, 2019 in accordance with the fair value hierarchy: September 30, 2020 September 30, 2019 (in thousands) Level 1 Level 2 Level 3 Level 1 Level 2 Level 3 Assets Cash equivalents $ 209,421 $ — $ — $ 72,132 $ — $ — Forward currency contracts — 2,209 — — 3,420 — Liabilities Forward currency contracts — $ 102 — — $ — — The Company's remaining financial instruments consist primarily of cash, accounts receivable and accounts payable whose carrying value approximate their fair value due to their short-term nature. The estimated fair value of financial instruments not carried at fair value in the consolidated balance sheets were as follows: September 30, 2020 September 30, 2019 (in thousands) Carrying Value Fair Value Carrying Value Fair Value First Lien Term Loan Facility due December 22, 2023 $ 812,120 $ 808,060 $ 852,120 $ 853,543 Total debt $ 812,120 $ 808,060 $ 852,120 $ 853,543 In determining the approximate fair value of its long-term debt, the Company used the trading value among financial institutions, which were classified within Level 2 of the fair value hierarchy. The carrying value of the ABL Credit Facility approximates fair value due to it being market-linked variable rate debt.

COMMITMENTS AND CONTINGENCIES

COMMITMENTS AND CONTINGENCIES	12 Months Ended
	Sep. 30, 2020
Commitments and Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	17. COMMITMENTS AND CONTINGENCIES The Company has obligations related to commitments to purchase certain goods. As of September 30, 2020 , such obligations were $130,932 for fiscal 2021 , $1,828 for fiscal 2022 and $13 thereafter. These amounts represent open purchase orders for materials used in production. Insurable Liabilities — The Company maintains policies with various insurance companies for its workers' compensation, product, property, general, auto, and executive liability risks. The insurance policies that the Company maintains have various retention levels and excess coverage limits. The establishment and update of liabilities for unpaid claims, including claims incurred but not reported, is based on management's estimate as a result of the assessment by the Company's claim administrator of each claim and an independent actuarial valuation of the nature and severity of total claims. The Company utilizes a third-party claims administrator to pay claims, track and evaluate actual claims experience, and ensure consistency in the data used in the actuarial valuation. Legal Contingencies — The Company is a defendant in a number of pending legal proceedings, some of which were inherited from its former parent, Tyco International Ltd. ("Tyco"), including certain product liability claims. Several lawsuits have been filed against the Company and the Company has also received other claim demand letters alleging that the Company's anti-microbial coated steel sprinkler pipe, which the Company has not manufactured or sold for several years, is incompatible with chlorinated polyvinyl chloride and caused stress cracking in such pipe manufactured by third parties when installed together in the same sprinkler system, which the Company refers to collectively as the " Special Products Claims ." After an analysis of claims experience, the Company reserved its best estimate of the probable and reasonably estimable losses related to these matters. The Company's total product liability reserves for Special Products Claims and other product liability matters were $597 and $2,424 as of September 30, 2020 and September 30, 2019 , respectively. As of September 30, 2020 , the Company believes that the range of reasonably possible losses for Special Products Claims and other product liabilities is between $1,000 and $8,000 . During fiscal 2019, after a court judgment was issued in one case between the Company and Tyco regarding the indemnification of expenses, fees and settlement amounts relating to the incompatibility issue, the Company and Tyco entered into a global settlement regarding the issue. The Company agreed to fund the total settlement in exchange for Tyco's agreement to cap the Company's Special Products Claim deductible at $12,000 , as opposed to the $13,000 cap negotiated within the original indemnity agreement. In conjunction with the payment of that settlement, Tyco and the Company examined the Company's total Special Products Claim payments and agreed that with that settlement payment and payment of a few other legal fee invoices, all of which have now been paid, the Company had met its $12,000 deductible obligation related to these Special Products Claims. Tyco, now Johnson Controls, Inc. ("JCI"), has a contractual obligation to indemnify the Company in respect of all remaining and future claims of incompatibility between the Company's antimicrobial coated steel sprinkler pipe and CPVC pipe used in the same sprinkler system. Tyco has defended and indemnified the Company on Special Products Claims as required. At this time, the Company does not expect the outcome of the Special Products Claims proceedings, either individually or in the aggregate, to have a material adverse effect on its business, financial condition, results of operations or cash flows, and the Company believes that its reserves are adequate for all remaining contingencies for Special Products Claims. During fiscal 2017, the U.S. Department of Commerce ruled on a scope request in relation to an Antidumping Duty Order for Malleable Iron Pipe Fittings from China. The ruling subjected certain of the Company's imports of conduit fittings within the Atkore Steel Components Inc. ("ASCI") business (acquired in November 2014) to antidumping duties, which are incremental to the duties previously paid upon importation. The Company appealed the scope decision and established an accrual of $7,501 during the second quarter of fiscal 2017 for the related contingent liability with the related expense recorded in selling, general and administrative expenses in the Company's consolidated statements of operations which covers the post-acquisition period through the date of the scope ruling. On appeal, the Court of International Trade ordered the U.S. Department of Commerce to re-examine certain aspects of its prior scope ruling, and after doing so, the Department filed an amended scope ruling for the Court of International Trade’s consideration. The amended scope ruling finds the ASCI products at issue in the matter to be outside the scope of the aforesaid Antidumping Duty Order. On August 3, 2018, the Court of International Trade sustained the U.S. Department of Commerce’s new scope determination and entered judgment thereupon. The U.S. Department of Commerce did not appeal the ruling by the Court of International Trade by the October 2, 2018 deadline and in fiscal 2018, the Company reversed $7,171 of the related accrual established during the second quarter of fiscal 2017. During the year ended September 30, 2020, one of the Company’s manufacturing facilities experienced a flood which resulted in damages to certain property, plant and equipment. This facility is covered under the Company’s property and casualty loss and business interruption insurance policies. The Company has recorded an expense of $6,046 for losses related to fully written off property, plant and equipment, with a related insurance recovery of $5,046 within selling, general and administrative expenses in its consolidated statements of operations for the year ended September 30, 2020. The Company has begun to receive reimbursement and believes that, other than the $ 1,000 deductible expense under the related insurance claim, it will be reimbursed for substantially all other property, plant and equipment losses in connection with the event under its current insurance policies. In addition to the matters discussed above, from time to time, the Company is subject to a number of disputes, administrative proceedings and other claims arising out of the ordinary conduct of the Company's business. These matters generally relate to disputes arising out of the use or installation of the Company's products, product liability litigation, contract disputes, patent infringement accusations, employment matters, personal injury claims and similar matters. On the basis of information currently available to the Company, it does not believe that existing proceedings and claims will have a material adverse effect on its business, financial condition, results of operations or cash flows. However, litigation is unpredictable, and the Company could incur judgments or enter into settlements for current or future claims that could adversely affect its business, financial condition, results of operations or cash flows.

GUARANTEES

GUARANTEES	12 Months Ended
	Sep. 30, 2020
Guarantees [Abstract]
GUARANTEES	18. GUARANTEES The Company has outstanding letters of credit totaling $9,501 supporting workers' compensation and general liability insurance policies and surety bonds primarily related to performance guarantees on supply agreements and construction contracts, and payment of duties and taxes totaling $19,468 as of September 30, 2020 . In disposing of assets or businesses, the Company often provides representations, warranties and indemnities to cover various risks including unknown damage to the assets, environmental risks involved in the sale of real estate, liability to investigate and remediate environmental contamination at waste disposal sites and manufacturing facilities, and unidentified tax liabilities and legal fees related to periods prior to disposition. The Company does not have the ability to estimate the potential liability from such indemnities because they relate to unknown conditions. However, the Company has no reason to believe that these uncertainties would have a material adverse effect on the Company's business, financial condition, results of operations or cash flows.

RELATED PARTY TRANSACTIONS

RELATED PARTY TRANSACTIONS	12 Months Ended
	Sep. 30, 2020
Related Party Transactions [Abstract]
RELATED PARTY TRANSACTIONS	19. RELATED PARTY TRANSACTIONS In December 2010, CD&R Allied Holdings, L.P. (the "CD&R Investor") acquired a majority stake in the Company (the "CD&R Acquisition"). In connection with the CD&R Acquisition, the Company, AIH and AII entered into a consulting agreement (the "Consulting Agreement") with CD&R. On January 22, 2018, the Company announced a stock repurchase transaction whereby the Company agreed to repurchase from the CD&R Investor, a related party, approximately 17.2 million shares of the Company's common stock, par value $0.01 per share, at a per share price equal to $21.77 , for a total purchase price of $375,000 , subject to the terms and conditions set forth in the stock purchase agreement. Following the stock repurchase transaction in January 2018 and secondary offerings of the Company's common stock in February 2018 and May 2018, the CD&R Investor did not own any of the Company's common stock subsequent to May 2018. Prior to the CD&R Investor selling all of its remaining shares of the Company in May 2018, transactions between the Company, CD&R and affiliates of CD&R were considered to be between related parties. In addition to the consulting fees referenced above, affiliates of CD&R own equity positions in one of the Company's customers. Net sales during fiscal 2018 to this customer were $6,000

SEGMENT INFORMATION

SEGMENT INFORMATION	12 Months Ended
	Sep. 30, 2020
Segment Reporting [Abstract]
SEGMENT INFORMATION	20. SEGMENT INFORMATION The Company has two operating segments, which are also its reportable segments. The Company's operating segments are organized based upon primary market channels and, in most instances, the end use of products. Through its Electrical Raceway segment, the Company manufactures products that deploy, isolate and protect a structure's electrical circuitry from the original power source to the final outlet. These products, which include electrical conduit, armored cable, cable trays, mounting systems and fittings, are critical components of the electrical infrastructure for maintenance, repair and remodel markets. The vast majority of the Company's Electrical Raceway net sales are made to electrical distributors, who then serve electrical contractors and the Company considers both to be customers. Through the MP&S segment, the Company provides products and services that frame, support and secure component parts in a broad range of structures, equipment and systems in electrical, industrial and construction applications. The Company's principal products in this segment are metal framing products and in-line galvanized mechanical tube. Through its metal framing business, the Company designs, manufactures and installs metal strut and fittings used to assemble mounting structures that support heavy equipment and electrical content in buildings and other structures. Both segments use Adjusted EBITDA as the primary measure of profit and loss. Segment Adjusted EBITDA is the income (loss) before income taxes, adjusted to exclude unallocated expenses, depreciation and amortization, interest expense, net, gain (loss) on extinguishment of debt, restructuring charges, stock-based compensation, certain legal matters, transaction costs, gain on purchase of business, gain on sale of a business and other items, such as inventory reserves and adjustments, loss on disposal of property, plant and equipment, insurance recovery related to damages of property, plant and equipment, release of indemnified uncertain tax positions, and realized or unrealized gain (loss) on foreign currency impacts of intercompany loans and related forward currency derivatives. Intersegment transactions primarily consist of product sales at designated transfer prices on an arm's-length basis. Gross profit earned and reported within the segment is eliminated in the Company's consolidated results. Certain manufacturing and distribution expenses are allocated between the segments on a pro rata basis due to the shared nature of activities. Recorded amounts represent a proportional amount of the quantity of product produced for each segment. Certain assets, such as machinery and equipment and facilities, are not allocated to each segment despite serving both segments. These shared assets are reported within the MP&S segment. We allocate certain corporate operating expenses that directly benefit our operating segments, such as insurance and information technology, on a basis that reasonably approximates an estimate of the use of these services. Fiscal year ended September 30, 2020 September 30, 2019 September 30, 2018 (in thousands) External Net Sales Inter- segment Sales Adjusted EBITDA External Net Sales Inter- segment Sales Adjusted EBITDA External Net Sales Inter- segment Sales Adjusted EBITDA Electrical Raceway $ 1,328,721 $ 2,554 $ 299,485 $ 1,442,278 $ 1,215 $ 292,585 $ 1,365,067 $ 1,544 $ 255,260 MP&S 436,700 — $ 61,152 474,260 — $ 70,040 470,072 81 $ 51,339 Eliminations — (2,554 ) — (1,215 ) — (1,625 ) Consolidated operations $ 1,765,421 $ — $ 1,916,538 $ — $ 1,835,139 $ — Capital Expenditures Total Assets (in thousands) September 30, 2020 September 30, 2019 September 30, 2018 September 30, 2020 September 30, 2019 September 30, 2018 Electrical Raceway $ 17,667 $ 19,856 $ 16,389 $ 835,629 $ 871,771 $ 751,024 MP&S 13,870 13,934 14,267 215,350 252,708 291,164 Unallocated 2,233 1,070 7,845 507,546 312,516 281,872 Consolidated operations $ 33,770 $ 34,860 $ 38,501 $ 1,558,525 $ 1,436,995 $ 1,324,060 Presented below is a reconciliation of operating segment Adjusted EBITDA to Income before income taxes : Fiscal Year Ended (in thousands) September 30, 2020 September 30, 2019 September 30, 2018 Operating segment Adjusted EBITDA Electrical Raceway $ 299,485 $ 292,585 $ 255,260 MP&S 61,152 70,040 51,339 Total $ 360,637 $ 362,625 $ 306,599 Unallocated expenses (a) (34,002 ) (38,217 ) (35,050 ) Depreciation and amortization (74,470 ) (72,347 ) (66,890 ) Interest expense, net (40,062 ) (50,473 ) (40,694 ) Gain (loss) on extinguishment of debt (273 ) — — Restructuring charges (3,284 ) (3,804 ) (1,849 ) Stock-based compensation (13,064 ) (11,798 ) (14,664 ) Certain legal matters — — 4,833 Transaction costs (196 ) (1,200 ) (9,314 ) Gain on purchase of business — 7,384 — Gain on sale of a business — — 27,575 Other (b) 6,712 (7,501 ) (4,194 ) Income before income taxes $ 201,998 $ 184,669 $ 166,352 (a) Represents unallocated selling, general and administrative activities and associated expenses including, in part, executive, legal, finance, human resources, information technology, business development and communications, as well as certain costs and earnings of employee-related benefits plans, such as stock-based compensation and a portion of self-insured medical costs. (b) Represents other items, such as inventory reserves and adjustments, loss on disposal of property, plant and equipment, insurance recovery related to damages of property, plant and equipment, release of indemnified uncertain tax positions and realized or unrealized gain (loss) on foreign currency impacts of intercompany loans and related forward currency derivatives. The Company's long-lived assets and net sales by geography were as follows: Long-lived assets Net sales (in thousands) September 30, 2020 September 30, 2019 September 30, 2018 September 30, 2020 September 30, 2019 September 30, 2018 United States $ 203,694 $ 219,614 $ 201,101 $ 1,563,258 $ 1,689,194 $ 1,651,636 Other Americas 146 147 138 26,421 33,485 43,013 Europe 41,283 43,207 11,090 132,299 142,279 90,915 Asia-Pacific 1,759 1,998 2,386 43,443 51,580 49,575 Total $ 246,882 $ 264,966 $ 214,715 $ 1,765,421 $ 1,916,538 $ 1,835,139 The table below shows the amount of net sales from external customers for each of the Company's product categories which accounted for 10% or more of consolidated net sales in any of the last three fiscal years: Fiscal Year Ended (in thousands) September 30, 2020 September 30, 2019 September 30, 2018 Metal Electrical Conduit and Fittings $ 484,476 $ 546,533 $ 517,935 Armored Cable and Fittings 306,460 360,494 336,388 PVC Electrical Conduit & Fittings 308,561 292,243 311,811 Cable Tray & Cable Ladders 193,306 199,303 145,795 Other raceway products 35,918 43,705 53,138 Electrical Raceway 1,328,721 1,442,278 1,365,067 Mechanical Pipe 244,902 259,613 253,381 Other MP&S products 191,798 214,647 216,691 MP&S 436,700 474,260 470,072 Net sales $ 1,765,421 $ 1,916,538 $ 1,835,139 Risks and Concentrations Concentration of Credit Risk — The Company extends credit to various customers in the retail and construction industries. Collection of trade receivables may be affected by changes in economic or other industry conditions and may, accordingly, impact the Company's overall credit risk. Although the Company generally does not require collateral, the Company performs ongoing credit evaluations of customers and maintains reserves for potential credit losses. As of September 30, 2020, one customer, Sonepar Management US, Inc., represented 11% of the Company's accounts receivable balance due to increased sales in the last 60 days of the year. As of September 30, 2019, no single customer represented more than 10% of the Company's accounts receivable balance. For fiscal 2020, 2019 and 2018, no single customer accounted for more than 10% of sales. Concentration of Employees — As of September 30, 2020 , approximately 24% of the Company's employees were represented by a union under a collective bargaining agreement. All unions are located in either the United States or Canada, with no unions or Worker's Councils at any of the other locations abroad. On July 14, 2020, the Company and the United Steelworkers Union, representing approximately 350 employees, reached agreement on the terms of a new collective bargaining agreement for our largest facility in Harvey, Illinois, which expires in April 2024.

QUARTERLY OPERATING RESULTS (UN

QUARTERLY OPERATING RESULTS (UNAUDITED)	12 Months Ended
	Sep. 30, 2020
Quarterly Financial Information Disclosure [Abstract]
QUARTERLY OPERATING RESULTS (UNAUDITED)	21. QUARTERLY OPERATING RESULTS (UNAUDITED) The following table presents unaudited selected quarterly financial data for the years ended September 30, 2020 and September 30, 2019 . The operating results for any quarter are not necessarily indicative of the results of any future period. Three Months Ended (in thousands, except per share data) December 27, 2019 March 27, 2020 June 26, 2020 September 30, 2020 December 29, 2018 March 30, 2019 June 29, 2019 September 30, 2019 (1) Net sales $ 447,448 $ 455,654 $ 384,899 $ 477,420 $ 452,028 $ 469,309 $ 493,491 $ 501,710 Gross profit 116,844 131,603 95,813 147,054 110,256 117,088 126,134 143,722 Net income 34,790 39,193 24,078 54,241 26,949 29,555 36,550 45,997 Net income per share Basic (2) $ 0.72 $ 0.81 $ 0.50 $ 1.12 $ 0.56 $ 0.62 $ 0.77 $ 0.96 Diluted (2) 0.71 0.80 0.49 1.11 0.54 0.61 0.75 0.94 (1) Includes a $7,384 gain on the acquisition of Cor-Tek. See Note 3, "Acquisitions" for additional information. (2) The sum of the quarters may not equal the total of the respective year's earnings per share due to changes in the weighted average shares outstanding throughout the year.

SUBSEQUENT EVENTS

SUBSEQUENT EVENTS	12 Months Ended
	Sep. 30, 2020
Subsequent Events [Abstract]
SUBSEQUENT EVENTS	22. SUBSEQUENT EVENTS On October 22, 2020, the Company acquired the assets of Queen City Plastics, Inc., a manufacturer of PVC conduit, elbows and fittings for the electrical market for an estimated purchase price of $ 7.3 million. Subsequent to September 30, 2020, the Company repurchased 637,200 shares for $ 15.0 million reducing the Company's share count to 47,409,886 as of November 13, 2020

SCHEDULE I - CONDENSED FINANCIA

SCHEDULE I - CONDENSED FINANCIAL INFORMATION	12 Months Ended
	Sep. 30, 2020
Condensed Financial Information Disclosure [Abstract]
SCHEDULE I - CONDENSED FINANCIAL INFORMATION	SCHEDULE I ATKORE INTERNATIONAL GROUP INC. (PARENT) CONDENSED FINANCIAL INFORMATION CONDENSED BALANCE SHEETS (in thousands, except share and per share data) September 30, 2020 September 30, 2019 Assets Investment in subsidiary $ 378,410 $ 232,936 Total Assets 378,410 232,936 Liabilities and Equity Total Liabilities $ — $ — Equity: Common stock, $0.01 par value, 1,000,000,000 shares authorized, 47,407,023 and 46,955,163 shares issued and outstanding, respectively $ 475 $ 471 Treasury stock, held at cost, 260,900 and 260,900 shares, respectively (2,580 ) (2,580 ) Additional paid-in capital 487,223 477,139 Accumulated deficit (64,154 ) (200,396 ) Accumulated other comprehensive loss (42,554 ) (41,698 ) Total Equity 378,410 232,936 Total Liabilities and Equity $ 378,410 $ 232,936 See Notes to Condensed Financial Information SCHEDULE I ATKORE INTERNATIONAL GROUP INC. (PARENT) CONDENSED FINANCIAL INFORMATION CONDENSED STATEMENTS OF OPERATIONS Fiscal Year Ended (in thousands) September 30, 2020 September 30, 2019 September 30, 2018 Equity in net income of subsidiary $ 152,302 $ 139,051 $ 136,645 Net income 152,302 139,051 136,645 Other comprehensive (loss) income of subsidiary, net of tax (856 ) (22,927 ) 1,544 Comprehensive income $ 151,446 $ 116,124 $ 138,189 See Notes to Condensed Financial Information SCHEDULE I ATKORE INTERNATIONAL GROUP INC. (PARENT) CONDENSED FINANCIAL INFORMATION CONDENSED STATEMENTS OF CASH FLOWS For the Year Ended (in thousands) September 30, 2020 September 30, 2019 September 30, 2018 Cash Flows from Operating Activities: Net cash provided by operating activities $ — $ — $ — Cash Flows from Investing Activities: Distribution received from subsidiary 17,983 24,419 411,775 Distribution paid to subsidiary — (7,374 ) (20,110 ) Net cash provided by investing activities 17,983 17,045 391,665 Cash Flows from Financing Activities: Issuance of common stock, net of taxes withheld (2,972 ) 7,374 20,110 Repurchase of common shares (15,011 ) (24,419 ) (411,775 ) Net cash used in financing activities (17,983 ) (17,045 ) (391,665 ) Net change in cash and cash equivalents — — — Cash and cash equivalents: Beginning — — — Ending $ — $ — $ — See Notes to Condensed Financial Information SCHEDULE I ATKORE INTERNATIONAL GROUP INC. (PARENT) CONDENSED FINANCIAL INFORMATION NOTES TO CONDENSED FINANCIAL INFORMATION (dollars in thousands) 1. Description of Atkore International Group Inc. Atkore International Group Inc. (the "Company," "Parent" or "Atkore") was incorporated in the State of Delaware on November 4, 2010. The Company is the stockholder of Atkore International Holdings Inc. ("AIH"), which is the sole stockholder of Atkore International, Inc. ("AII"). Prior to the transactions described below, all of the capital stock of AII was owned by Tyco International Ltd. ("Tyco"). The business of AII was operated as the Tyco Electrical and Metal Products ("TEMP") business of Tyco. Atkore was initially formed by Tyco as a holding company to hold ownership of TEMP. On November 9, 2010, Tyco announced that it had entered into an agreement to sell a majority interest in TEMP to CD&R Allied Holdings, L.P. (the "CD&R Investor), an affiliate of the private equity firm Clayton Dubilier & Rice, LLC ("CD&R"). On December 22, 2010, the transaction was completed and CD&R acquired shares of a newly created class of cumulative convertible preferred stock (the "Preferred Stock") of the Company. The Preferred Stock initially represented 51% of the Company's outstanding capital stock (on an as-converted basis). On December 22, 2010, the Company also issued common stock (the "Common Stock") to Tyco's wholly owned subsidiary, Tyco International Holding S.à.r.l. ("Tyco Seller"), that initially represented the remaining 49% of the Company's outstanding capital stock. Subsequent to December 22, 2010, the Company has operated as an independent, stand-alone entity. On March 6, 2014, the Company entered into a non-binding letter of intent (the "Letter of Intent") with Tyco for the acquisition (the "Acquisition") of 40.3 million shares of Common Stock held by Tyco Seller. On April 9, 2014, the Company paid $250,000 to Tyco Seller to redeem the shares, which were subsequently retired. The Company paid $2,000 of expenses related to the share redemption. In a separate transaction on the same date, the CD&R Investor converted its Preferred Stock and accumulated Preferred Dividends into Common Stock. As a result, Common Stock is the Company's sole issued and outstanding class of securities. The Parent has no significant operations or assets other than its indirect ownership of the equity of AII. Accordingly, the Parent is dependent upon distributions from AII to fund its obligations. However, under the terms of the agreements governing AII's borrowings, AII's ability to pay dividends or lend to Atkore Holding or the Parent, is restricted. While certain exceptions to the paying dividends or lending funds restrictions exist, these restrictions have resulted in the restricted net assets (as defined in Rule 4-08(e)(3) of Regulation S-X) of the Company's subsidiaries exceeding 25% of the consolidated net assets of the Company and its subsidiaries. Atkore Holding has no obligations to pay dividends to the Parent except to pay specified amounts to Parent in order to fund the payment of the Parent's tax obligations. 2. Basis of Presentation The accompanying condensed Parent only financial statements are required in accordance with Rule 4-08(e)(3) of Regulation S-X. The financial statements include the amounts of the Parent and its investment in its subsidiaries under the equity method and does not present the financial statements of the Parent and its subsidiaries on a consolidated basis. Under the equity method, investment in its subsidiaries is stated at cost plus contributions and equity in undistributed income (loss) of subsidiary less distributions received since the date of acquisition. These condensed Parent only financial statements should be read in conjunction with the Atkore International Group Inc. consolidated financial statements and their accompanying notes. 3. Dividends and Distributions from Subsidiaries The Company received distributions of $17,983 , $24,419 , and $411,775 from its subsidiaries for the years ended September 30, 2020 , September 30, 2019 and September 30, 2018 , respectively. The distributions received in fiscal 2019, 2018 and 2017 were used to repurchase shares of the Company's common stock. These dividends were permissible under an exception to the net asset restrictions of the agreements governing AII's borrowings, which allow for dividend payments from AII to AIH or the Parent for the purpose of repurchasing shares of Parent's common stock.

SCHEDULE II - VALUATION AND QUA

SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS	12 Months Ended
	Sep. 30, 2020
SEC Schedule, 12-09, Valuation and Qualifying Accounts [Abstract]
SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS	SCHEDULE II—VALUATION AND QUALIFYING ACCOUNTS (in thousands) Balance at Beginning of Year Additional (Charges)/Benefit to Income Write offs and Other Balance at End of Year Accounts Receivable Allowance for Doubtful Accounts: For the fiscal year ended: 2020 $ (2,608 ) (1,990 ) 1,431 $ (3,167 ) 2019 $ (1,762 ) (970 ) 124 $ (2,608 ) 2018 $ (1,239 ) (389 ) (134 ) $ (1,762 ) Deferred Tax Valuation Allowance: For the fiscal year ended: 2020 $ (7,764 ) (2,047 ) (392 ) $ (10,203 ) 2019 $ (7,427 ) (346 ) 9 $ (7,764 ) 2018 $ (9,512 ) 428 1,657 $ (7,427 )

BASIS OF PRESENTATION AND SUM_2

BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	12 Months Ended
	Sep. 30, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation — The accompanying audited consolidated financial statements of the Company and all of its subsidiaries included herein have been prepared in accordance with accounting principles generally accepted in the United States of America ("GAAP"). The audited consolidated financial statements include the assets and liabilities used in operating the Company's business. All intercompany balances and transactions have been eliminated in consolidation. The results of companies acquired or disposed of are included in the audited consolidated financial statements from the effective date of acquisition or up to the date of disposal.
Fiscal Periods	Fiscal Periods — The Company has a fiscal year that ends on September 30. The Company's fiscal quarters typically end on the last Friday in December, March and June as it follows a 4-5-4 calendar.
Use of Estimates	Use of Estimates — The preparation of the consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclose contingent assets and liabilities at the date of the consolidated financial statements and report the associated amounts of revenues and expenses. Actual results could differ materially from these estimates.
Revenue Recognition	Revenue Recognition — The Company’s revenue arrangements primarily consist of a single performance obligation to transfer promised goods which is satisfied at a point in time when title, risks and rewards of ownership, and subsequently control have transferred to the customer. This generally occurs when the product is shipped to the customer, with an immaterial amount of transactions in which control transfers upon delivery. The Company primarily offers assurance-type standard warranties that do not represent separate performance obligations. The Company has certain arrangements that require it to estimate at the time of sale the amounts of variable consideration that should not be recorded as revenue as certain amounts are not expected to be collected from customers, as well as an estimate of the value of products to be returned. The Company principally relies on historical experience, specific customer agreements, and anticipated future trends to estimate these amounts at the time of sale and to reduce the transaction price. These arrangements include sales discounts and allowances, volume rebates, and returned goods. The Company has elected to utilize certain practical expedients available under GAAP. The Company records amounts billed to customers for reimbursement of shipping and handling costs within revenue. Shipping and handling costs associated with outbound freight after control over a product has transferred to a customer are accounted for as fulfillment costs and are included in cost of sales. Sales taxes and other usage-based taxes are excluded from revenue. The practical expedient not to disclose information about remaining performance obligations has also been elected as these obligations have an original duration of one year or less. The Company does not evaluate whether the selling price includes a financing interest component for contracts that are less than a year. The Company also expenses costs incurred to obtain a contract, primarily sales commissions, as all obligations will be settled in less than one year. The Company typically receives payment 30 to 60 days from the point it has satisfied the related performance obligation. See Note 20, ''Segment Information'' for revenue disaggregated by geography and product categories.
Cost of Sales	Cost of Sales — The Company includes all costs directly related to the production of goods for sale in cost of sales in the statement of operations. These costs include direct material, direct labor, production related overheads, excess and obsolescence costs, lower of cost or market provisions, freight and distribution costs, and the depreciation and amortization of assets directly used in the production of goods for sale.
Selling, General and Administrative Expenses	Selling, General and Administrative Expenses — These amounts primarily include payroll-related expenses for both administrative and selling personnel, compensation expense from stock-based awards, restructuring-related charges, third-party professional services and transactional gains or losses for foreign currency transactions, excluding the foreign exchange exposure for intercompany loan transactions, which is included in Other income, net .
Cash and Cash Equivalents	Cash and Cash Equivalents — The Company considers all highly liquid investments with a maturity of three months or less, when purchased, to be cash equivalents.
Accounts Receivable and Allowance for Doubtful Accounts	Accounts Receivable and Allowance for Doubtful Accounts — The Company carries its accounts receivable at their face amounts less an allowance for doubtful accounts. The allowance for doubtful accounts receivable reflects the best estimate of losses inherent in the Company's accounts receivable portfolio determined on the basis of historical experience, specific allowances for known troubled accounts and other available evidence.
Inventories	Inventories
Property, Plant and Equipment	Property, Plant and Equipment — Property, plant and equipment, net, is recorded at cost less accumulated depreciation. Maintenance and repair expenditures are charged to expense when incurred. Depreciation is calculated using the straight-line method over the estimated useful lives of the related assets as follows: Buildings 4 to 40 years Building improvements 3 to 20 years Machinery and equipment 1 to 20 years Leasehold improvements Lesser of remaining term of the lease or useful life Software 2 to 10 years The internal and external costs incurred to develop internal use computer software during the application development stage of the implementation, including the design of the chosen path, are capitalized. Other costs, including expenses incurred during the preliminary project stage, training expenses, data conversion costs and expenses incurred in the post implementation stage are expensed in the period incurred. Capitalized costs are amortized ratably over the useful life of the software when the software becomes operational. Upgrades and enhancements to internal use software are capitalized only if the costs result in additional functionality. The Company does not plan to sell or market our internal use computer software to third parties.
Long-Lived Asset Impairments	Long-Lived Asset and Finite - Lived Intangible Asset Impairments — The Company reviews long-lived assets, including property, plant and equipment and finite-lived intangible assets for impairment whenever events or changes in business circumstances indicate that the carrying amount of the asset may not be fully recoverable. The Company groups assets at the lowest level for which cash flows are separately identified in order to measure an impairment. Recoverability of an asset or asset group is first measured by a comparison of the carrying amount to its estimated undiscounted future cash flows expected to be generated by the asset or asset group. If the carrying amount exceeds its estimated undiscounted future cash flows, an impairment charge is recognized as the amount by which the carrying amount of the asset or asset group exceeds the estimated fair value. If impairment is determined to exist, any related impairment loss is calculated based on the estimated fair value. Impairment losses on assets to be disposed of or held for sale, if any, are based on the estimated proceeds to be received, less costs of disposal. The Company also considers potential impairment indicators associated with other finite-lived intangible assets, including its customer relationships, patents, and non-compete agreements. An impairment is recognized if the carrying value of an asset or asset group exceeds the estimated undiscounted future cash flows expected to result from the use of the asset or asset group and its eventual disposition. The Company's key customers are primarily wholesale and national distributors. The terms of these relationships are based on purchase orders and are not contractually based. Customer relationships are amortized on a straight-line basis over their useful lives, ranging from 6 to 14 years. The Company evaluates the appropriateness of remaining useful lives based on customer attrition rates. Other intangible assets are amortized on a straight-lined basis over their estimated useful lives, ranging from 1 to 20
Goodwill and Indefinite-Lived Intangible Assets Impairments	Goodwill and Indefinite-Lived Intangible Asset Impairments — The Company assesses the recoverability of goodwill and indefinite-lived trade names on an annual basis in accordance with Accounting Standards Codification ("ASC") 350 "Intangibles - Goodwill and Other." The measurement date is the first day of the fourth fiscal quarter, or more frequently, if events or circumstances indicate that it is more likely than not that the fair value of a reporting unit or the respective indefinite-lived trade name is less than the carrying value. The Company can elect to perform a quantitative or qualitative test of impairment. For fiscal 2020, 2019 and 2018 Company performed a quantitative impairment assessment for goodwill. Additionally, the economic impacts of the COVID-19 outbreak were more substantial for the Company’s reporting unit related to its operations in Europe, Middle East, and Africa (EMEA), due to the cross-border transactions that occur within those operations, the cross-border interactions needed to fully realize the synergies associated with recent in-region acquisitions and the COVID-19 related closing of these borders. The Company deemed the economic related developments of the outbreak to be a triggering event for the EMEA reporting unit as of March 27, 2020 and performed an interim goodwill impairment test as of that date. The Company calculated the fair value of its six reporting units considering three valuation approaches: (a) the income approach; (b) the guideline public company method; and (c) the comparable transaction method. The income approach calculates the fair value of the reporting unit using a discounted cash flow approach. Internally forecasted future cash flows, which the Company believes reasonably approximate market participant assumptions, are discounted using a weighted average cost of capital (Discount Rate) developed for each reporting unit. The Discount Rate is developed using market observable inputs, as well as considering whether or not there is a measure of risk related to the specific reporting unit’s forecasted performance. The key uncertainties in these calculations are the assumptions used in determining the reporting unit’s forecasted future performance, including revenue growth and EBITDA margins, as well as the perceived risk associated with those forecasts, along with selecting representative market multiples. Fair value under the guideline public company method is determined for each reporting unit by applying market multiples for comparable public companies to the reporting unit’s financial results. Fair value under the comparable transaction method is determined based on exchange prices in actual transactions and on asking prices for controlling interests in public or private companies currently offered for sale by applying market multiples for comparable public companies to the unit’s financial results. The key uncertainties in the guideline public company method and the comparable transaction method calculations are the assumptions used in determining the reporting unit's comparable public companies, comparable transactions and the selection of the market multiples. The Company did no t record any goodwill impairments in fiscal 2020, 2019 or 2018. As noted above, ASC 350 also requires that the Company test the indefinite-lived intangible assets for impairment at least annually. Under ASC 350, if the carrying value of the indefinite-lived asset is higher than its fair value, then the asset is deemed to be impaired and the impairment charge is estimated as the excess carrying value over the fair value. The Company calculated the fair value of its indefinite-lived intangible assets using the income approach, specifically the relief-from-royalty method. The relief-from-royalty method is used to estimate the cost savings that accrue to the owner of an intangible asset who would otherwise have to pay royalties or license fees on revenues earned through the use of the asset. Internally forecasted revenues, which the Company believes reasonably approximate market participant assumptions, are multiplied by a royalty rate to arrive at the estimated net after tax cost savings. The royalty rate used in the analysis is based on an analysis of empirical, market-derived royalty rates for guideline intangible assets. The net after tax cost savings are discounted using the Discount Rate. The Discount Rate is developed using market observable inputs, as well as considering whether or not there is a measure of risk related to the specific indefinite lived intangible assets' forecasted performance. The key uncertainties in these calculations are the assumptions used in determining the revenue associated with each indefinite-lived intangible asset and the royalty rate.
Fair Value Measurements	Fair Value Measurements — Authoritative guidance for fair value measurements establishes a three-level hierarchy that ranks the quality and reliability of information used in developing fair value estimates. The hierarchy gives the highest priority to quoted prices in active markets and the lowest priority to unobservable data. In cases where two or more levels of inputs are used to determine fair value, a financial instrument's level is determined based on the lowest level input that is considered significant to the fair value measurement in its entirety. The three levels of the fair value hierarchy are summarized as follows: Level 1-inputs are based upon quoted prices (unadjusted) in active markets for identical assets or liabilities which are accessible as of the measurement date. Level 2-inputs are based upon quoted prices for similar assets or liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, and model-derived valuations for the asset or liability that are derived principally from or corroborated by market data for which the primary inputs are observable, including forward interest rates, yield curves, credit risk and exchange rates. Level 3-inputs for the valuations are unobservable and are based on management's estimates of assumptions that market participants would use in pricing the asset or liability. The fair values are therefore determined using model-based techniques such as option pricing models and discounted cash flow models.
Income Taxes and Uncertain Tax Positions	Income Taxes and Uncertain Tax Positions — The Company accounts for income taxes under the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the consolidated financial statements. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and tax basis of assets and liabilities using enacted tax rates in effect for the year it is expected the differences will reverse. The effect of a change in tax rates on deferred tax assets and liabilities is recognized in income in the period of the enactment date. The Company periodically assesses the realizability of the deferred tax assets. In making this determination management considers all available evidence, both positive and negative, including earnings history, expectations of future taxable income and available tax planning strategies. A valuation allowance is recorded to reduce the Company's deferred tax assets to the amount that is considered more likely than not to be realized. Changes in the required valuation allowance are recorded in income in the period such determination is made. Certain tax positions may be considered uncertain requiring an assessment of whether an allowance should be recorded. Provisions for uncertain tax positions provide a recognition threshold based on an estimate of whether it is more likely than not that a position will be sustained upon examination. The Company measures its uncertain tax positions as the largest amount of benefit that is greater than a 50%
Leases	Leases — For fiscal 2019 and 2018, under ASC 840, all leases that are determined not to meet any of the capital lease criteria are classified as operating leases. Operating lease payments are recognized as an expense in the statement of operations on a straight-line basis over the lease term. For fiscal 2020, as a result of the adoption of ASC 842, the Company recognizes if an arrangement is a lease at the inception of the contract. The Company determines which party has the right to control an asset during the contract term and recognizes a Right of Use ("ROU") asset and lease obligations based on the present value of the future minimum lease payments over the term of the lease. Refer to Note 2 for further discussion of the Company’s accounting policy for leases. The Company elected the package of practical expedients permitted under the transition guidance within Topic 842, which allowed us to carry forward prior conclusions about lease identification, classification and initial direct costs for leases entered into prior to adoption of Topic 842. Additionally, for leases with a term of 12 months or less, the Company elected the short-term lease exemption, which allowed us to not recognize ROU assets or lease liabilities for qualifying leases existing at transition and new leases we may enter into in the future. Leases with an initial term of 12 months or less are classified as short-term leases and are not recorded on the consolidated balance sheets. The lease expense for short-term leases is recognized on a straight-line basis over the lease term. The Company engages in leasing transactions to meet the needs of the business. The Company leases certain manufacturing facilities, warehouses and distribution centers, office space, forklifts, vehicles and other machinery and equipment. The determination to lease, rather than purchase, an asset is primarily contingent upon capital requirements, duration of the forecasted business investment, and asset availability. The Company determines if an arrangement is a lease at inception and all arrangements deemed to be leases are subject to an assessment to determine the classification between finance and operating leases. The Company's significant assumptions and judgments in determining whether a contract is or contains a lease include establishing whether the supplier has the ability to use other assets to fulfill its service or whether the terms of the agreement enable the Company to control the use of a dedicated property, plant and equipment asset during the contract term. In the majority of the Company's contracts where it must identify whether a lease is present, it is readily determinable that the Company controls the use of the assets and obtains substantially all of the economic benefit during the term of the contract. In those contracts where identification is not readily determinable, the Company has determined that the supplier has either the ability to use another asset to provide the service or the terms of the contract give the supplier the rights to operate the asset at its discretion during the term of the contract, in which case the arrangement would not constitute a lease. Right-of-use assets and lease obligations are recognized based on the present value of the future minimum lease payments over the lease term as of the commencement date. The Company’s lease agreements have terms that include both lease and non-lease components. Lease component fees are included in the present value of future minimum lease payments. Conversely, non-lease components are not subject to capitalization and are expensed as incurred. Per Topic 842, the contractual interest rate is used to calculate the present value of the future minimum lease payments. However, the majority of the Company’s leases do not provide an implicit rate. Therefore, the Company's significant assumption and judgments in determining the discount rate include determining the incremental borrowing rate. The Company’s incremental borrowing rates are based on the term of the lease, the economic environment of the lease and the effect of collateralization. The valuation of the ROU asset also includes lease payments made in advance of the lease commencement date and initial direct costs incurred to secure the lease and is reduced for lease incentives. The lease terms include options to extend or terminate the lease when it is reasonably certain the Company will exercise the options.
Translation of Foreign Currency	Translation of Foreign Currency — For the Company's non-U.S. subsidiaries that report in a functional currency other than United States dollars, assets and liabilities are translated into United States dollars using year-end exchange rates. Revenue and expenses are translated at the monthly average exchange rates in effect during the fiscal year. Foreign currency translation adjustments are included as a component of accumulated other comprehensive loss within the consolidated statements of comprehensive income .
Recent Accounting Pronouncements	A summary of recently adopted Accounting Standards Update ("ASU")s are as follows. Adoption dates are on the first day of the fiscal year indicated below, unless otherwise specified. Adopted Guidance ASU Description of ASU Impact to Atkore Note Adoption Date 2016-02 Leases (Topic 842) The ASU requires companies to use a "right of use" lease model that assumes that each lease creates an asset (the lessee's right to use the leased asset) and a liability (the future rent payment obligations), which should be reflected on a lessee's balance sheet to fairly represent the lease transaction and the lessee's related financial obligations with terms of more than 12 months. The Company adopted the guidance in the first quarter of 2020 using the modified retrospective method. See Note 2, "Leases" for further detail. 2 2020 2018-14 Disclosure Framework - Changes to the Disclosure Requirements for Defined Benefit Plans The ASU amends ASC 715 to add, remove and clarify disclosure requirements related to defined benefit pension and other postretirement plans. The Company elected to adopt the guidance early in the quarter ended September 30, 2020. There was no material impact to the consolidated financial statements as a result of the adoption of ASU 2018-14. See Note 5, "Post Retirement Benefits" for further detail. 5 2020 A summary of guidance not yet adopted are as follows: Guidance not yet adopted ASU Description of ASU Impact to Atkore Effective Date 2016-13 Financial Instruments - Credit Losses (Topic 326) The ASU adds to U.S. GAAP an impairment model (known as the current expected credit loss (CECL) model) that is based on expected losses rather than incurred losses. Under the new guidance, an entity recognizes as an allowance its estimate of expected credit losses, which the FASB believes will result in more timely recognition of such losses. The Company adopted this standard in the first quarter of 2021. The adoption of the standard did not have a material impact on the Company’s consolidated financial statements. The Company implemented the appropriate changes to business processes and controls to support recognition and disclosure under the new standard. 2021 2019-12, Simplifying the accounting for income taxes (Topics 740) The ASU eliminates certain existing exceptions related to the general approach in ASC 740 relating to franchise taxes, reducing complexity in the interim period accounting for year to date loss limitations and changes in tax laws, and clarifying the accounting for transactions outside of business combination that result in a step up in the tax basis of goodwill. Under evaluation. Based on procedures performed to date, the Company does not anticipate the adoption of this ASU to be material to the financial statements. 2022 2020-04, Reference rate reform Topic 848: Facilitation of the effects of reference rate reform on financial reporting The ASU addresses constituents’ concerns about certain accounting consequences that could result from the global markets’ anticipated transition away from the use of the London Interbank Offered Rate (LIBOR) and other interbank offered rates to alternative reference rates. Under evaluation. 2022

BASIS OF PRESENTATION AND SUM_3

BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)	12 Months Ended
	Sep. 30, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of Estimated Useful Lives	Depreciation is calculated using the straight-line method over the estimated useful lives of the related assets as follows: Buildings 4 to 40 years Building improvements 3 to 20 years Machinery and equipment 1 to 20 years Leasehold improvements Lesser of remaining term of the lease or useful life Software 2 to 10 years As of September 30, 2020 and September 30, 2019 , property, plant and equipment at cost and accumulated depreciation were as follows: (in thousands) September 30, 2020 September 30, 2019 Land $ 20,460 $ 19,897 Buildings and related improvements 129,538 127,061 Machinery and equipment 332,260 318,421 Leasehold improvements 9,862 9,055 Software 27,028 24,835 Construction in progress 22,736 21,264 Property, plant and equipment 541,884 520,533 Accumulated depreciation (297,993 ) (259,830 ) Property, plant and equipment, net $ 243,891 $ 260,703
Schedule of New Accounting Pronouncements and Changes in Accounting Principles	A summary of recently adopted Accounting Standards Update ("ASU")s are as follows. Adoption dates are on the first day of the fiscal year indicated below, unless otherwise specified. Adopted Guidance ASU Description of ASU Impact to Atkore Note Adoption Date 2016-02 Leases (Topic 842) The ASU requires companies to use a "right of use" lease model that assumes that each lease creates an asset (the lessee's right to use the leased asset) and a liability (the future rent payment obligations), which should be reflected on a lessee's balance sheet to fairly represent the lease transaction and the lessee's related financial obligations with terms of more than 12 months. The Company adopted the guidance in the first quarter of 2020 using the modified retrospective method. See Note 2, "Leases" for further detail. 2 2020 2018-14 Disclosure Framework - Changes to the Disclosure Requirements for Defined Benefit Plans The ASU amends ASC 715 to add, remove and clarify disclosure requirements related to defined benefit pension and other postretirement plans. The Company elected to adopt the guidance early in the quarter ended September 30, 2020. There was no material impact to the consolidated financial statements as a result of the adoption of ASU 2018-14. See Note 5, "Post Retirement Benefits" for further detail. 5 2020 A summary of guidance not yet adopted are as follows: Guidance not yet adopted ASU Description of ASU Impact to Atkore Effective Date 2016-13 Financial Instruments - Credit Losses (Topic 326) The ASU adds to U.S. GAAP an impairment model (known as the current expected credit loss (CECL) model) that is based on expected losses rather than incurred losses. Under the new guidance, an entity recognizes as an allowance its estimate of expected credit losses, which the FASB believes will result in more timely recognition of such losses. The Company adopted this standard in the first quarter of 2021. The adoption of the standard did not have a material impact on the Company’s consolidated financial statements. The Company implemented the appropriate changes to business processes and controls to support recognition and disclosure under the new standard. 2021 2019-12, Simplifying the accounting for income taxes (Topics 740) The ASU eliminates certain existing exceptions related to the general approach in ASC 740 relating to franchise taxes, reducing complexity in the interim period accounting for year to date loss limitations and changes in tax laws, and clarifying the accounting for transactions outside of business combination that result in a step up in the tax basis of goodwill. Under evaluation. Based on procedures performed to date, the Company does not anticipate the adoption of this ASU to be material to the financial statements. 2022 2020-04, Reference rate reform Topic 848: Facilitation of the effects of reference rate reform on financial reporting The ASU addresses constituents’ concerns about certain accounting consequences that could result from the global markets’ anticipated transition away from the use of the London Interbank Offered Rate (LIBOR) and other interbank offered rates to alternative reference rates. Under evaluation. 2022

LEASES (Tables)

LEASES (Tables)	12 Months Ended
	Sep. 30, 2020
Leases [Abstract]
Assets and Liabilities	Leases (in thousands) September 30, 2020 Assets Operating lease assets $ 37,666 Finance lease assets 2,999 Total right-of-use assets, gross $ 40,665 Less: accumulated amortization (1,973 ) Right-of-use assets, net $ 38,692 Liabilities Current liabilities: Current portion of operating lease liabilities $ 15,356 Current portion of finance lease liabilities 430 Lease obligations $ 15,786 Noncurrent liabilities: Operating lease liabilities $ 23,354 Finance lease liabilities 789 Long-term lease obligations $ 24,143 Total lease obligations $ 39,929
Lease, Cost	The following table summarizes lease costs by type of cost for the fiscal year ended September 30, 2020 . In the consolidated statements of operations, cost of sales and selling, general and administrative expenses included lease costs of $14,295 and $2,714 respectively. (in thousands) Fiscal year ended September 30, 2020 Consolidated Statement of Operations Classification Total Amortization of right-of-use assets $ 14,803 Interest on lease liabilities 41 Variable lease costs 714 Short term lease costs 1,451 Total lease costs $ 17,009 Lease Term and Discount Rate Fiscal year ended September 30, 2020 Weighted-average remaining lease term (years) Operating leases 4.4 Finance leases 3.2 Weighted-average discount rate Operating leases 3.6 % Finance leases 3.1 % Other Information (in thousands) Fiscal year ended September 30, 2020 Cash paid for amounts included in the measurement of lease liabilities Operating cash flows from operating leases 12,939 Operating cash flows from finance leases 26 Financing cash flows from finance leases 500
Operating Lease, Liability, Maturity	The Company's maturity analysis of its lease liabilities as of fiscal year ended September 30, 2020 is as follows: (in thousands) Financing Leases Operating Leases 2021 $ 422 $ 12,902 2022 406 10,097 2023 332 7,228 2024 89 5,789 2025 3 3,451 2026 and after — 3,535 Total lease payments $ 1,252 $ 43,002 Less: Interest (33 ) (4,292 ) Present value of lease liabilities $ 1,219 $ 38,710
Finance Lease, Liability, Maturity	The Company's maturity analysis of its lease liabilities as of fiscal year ended September 30, 2020 is as follows: (in thousands) Financing Leases Operating Leases 2021 $ 422 $ 12,902 2022 406 10,097 2023 332 7,228 2024 89 5,789 2025 3 3,451 2026 and after — 3,535 Total lease payments $ 1,252 $ 43,002 Less: Interest (33 ) (4,292 ) Present value of lease liabilities $ 1,219 $ 38,710
Schedule of Minimum Future Operating Lease Payments	The following represents the Company's future minimum rental payments at September 30, 2019 , for agreements classified as operating leases under ASC 840 with non-cancelable terms in excess of one year: 2020 $ 13,526 2021 11,592 2022 8,666 2023 6,362 2024 5,097 2025 and thereafter 6,938 Total $ 52,181

ACQUISITIONS (Tables)

ACQUISITIONS (Tables)	12 Months Ended
	Sep. 30, 2020
Business Combinations [Abstract]
Schedule of Recognized Identified Assets Acquired and Liabilities Assumed	The following table summarizes the Level 3 fair values assigned to the net assets acquired and liabilities assumed as of the acquisition date for fiscal 2019: (in thousands) Vergokan Other Total Fair value of consideration transferred: Cash consideration $ 58,728 $ 41,641 $ 100,369 Other liability consideration — 1,400 1,400 Total consideration transferred 58,728 43,041 101,769 Fair value of assets acquired and liabilities assumed: Cash 829 1,541 2,370 Accounts receivable 8,761 8,217 16,978 Inventories 11,434 7,494 18,928 Intangible assets 12,621 16,400 29,021 Fixed assets 32,490 19,298 51,788 Accounts payable (18,716 ) (7,608 ) (26,324 ) Gain on purchase of business — (7,384 ) (7,384 ) Other 1,680 (3,412 ) (1,732 ) Net assets acquired 49,099 34,546 83,645 Excess purchase price attributed to goodwill acquired $ 9,629 $ 8,495 $ 18,124
Schedule of Finite-Lived Intangible Assets Acquired as Part of Business Combination	The following table summarizes the fair value of intangible assets as of the acquisition date: Vergokan Other ($ in thousands) Fair Value Weighted Average Useful Life (Years) Fair Value Weighted Average Useful Life (Years) Customer relationships $ 10,535 12.0 $ 15,400 10.0 Other 2,086 9.0 1,000 9.0 Total intangible assets $ 12,621 $ 16,400

DIVESTITURES (Tables)

DIVESTITURES (Tables)	12 Months Ended
	Sep. 30, 2020
Discontinued Operations and Disposal Groups [Abstract]
Sale Price Allocation	The cash consideration received, net assets divested and resulting gain on sale for the year ended September 30, 2018 are as follows: (in thousands) Flexhead Cash consideration $ 42,631 Net assets divested 15,056 Gain on sale of a business $ 27,575

POSTRETIREMENT BENEFITS (Tables

POSTRETIREMENT BENEFITS (Tables)	12 Months Ended
	Sep. 30, 2020
Retirement Benefits [Abstract]
Schedule of Net Periodic Benefit Cost	The net periodic (benefit)/cost for the periods presented was as follows: Fiscal Year Ended (in thousands) September 30, 2020 September 30, 2019 September 30, 2018 Interest cost 3,739 4,665 4,098 Expected return on plan assets (6,331 ) (6,372 ) (6,416 ) Amortization of actuarial loss 888 101 343 Net periodic (benefit) cost $ (1,704 ) $ (1,606 ) $ (1,975 ) The weighted-average assumptions used to determine net periodic pension (benefit) cost during the period were as follow: September 30, 2020 September 30, 2019 September 30, 2018 Discount rate 3.1 % 4.2 % 3.7 % Expected return on plan assets 6.3 % 6.3 % 6.4 % Rate of compensation increase N/a N/a N/a
Schedule of Change in Benefit Obligations and Plan Assets	The change in the benefit obligations, plan assets and the amounts recognized on the consolidated balance sheets was as follows (in thousands): Change in benefit obligations: Balance as of September 30, 2018 $ 120,957 Interest cost 4,665 Actuarial loss 17,463 Benefits and administrative expenses paid (5,125 ) Balance as of September 30, 2019 137,960 Interest cost 3,739 Actuarial loss 10,476 Benefits and administrative expenses paid (5,416 ) Balance as of September 30, 2020 $ 146,759 Change in plan assets: Balance as of September 30, 2018 104,454 Actual return on plan assets 3,043 Employer contributions 1,079 Benefits and administrative expenses paid (5,125 ) Balance as of September 30, 2019 103,451 Actual return on plan assets 6,587 Employer contributions 2,114 Benefits and administrative expenses paid (5,416 ) Balance as of September 30, 2020 $ 106,736 Funded status: Funded status as of September 30, 2019 $ (34,509 ) Funded status as of September 30, 2020 $ (40,023 )
Schedule of Amounts Recognized in Balance Sheet	(in thousands) September 30, 2020 September 30, 2019 Amounts recognized in the consolidated balance sheets consist of: Pension liabilities $ (40,023 ) $ (34,509 ) Net amount recognized $ (40,023 ) $ (34,509 ) Amounts recognized in accumulated other comprehensive loss (before income taxes) consist of: Net actuarial loss $ (42,367 ) $ (33,035 ) Total loss recognized $ (42,367 ) $ (33,035 ) Weighted-average assumptions used to determine pension benefit obligations at year end: Discount rate 2.5 % 3.1 % Rate of compensation increase N/a N/a
Schedule of Accumulated Benefit Obligations in Excess of Fair Value of Plan Assets	The following table summarizes the defined benefit pension plans with accumulated benefit obligations in excess of plan assets: (in thousands) September 30, 2020 September 30, 2019 Accumulated benefit obligation $ 146,759 $ 137,960 Fair value of plan assets 106,736 103,451 The following table summarizes the defined benefit pension plans with projected benefit obligations in excess of plan assets: (in thousands) September 30, 2020 September 30, 2019 Projected benefit obligation $ 146,759 $ 137,960 Fair value of plan assets 106,736 103,451
Schedule of Allocation of Plan Assets	Pension plans have the following weighted-average asset allocations: Asset Category: September 30, 2020 September 30, 2019 Equity securities 58% 58% Debt securities 39% 40% Cash and cash equivalents 3% 2% Total 100% 100% (in thousands) September 30, 2020 September 30, 2019 Level 1 Level 2 Total Level 1 Level 2 Total United States equity securities $ 30,738 $ 6,796 $ 37,534 $ 32,545 $ 6,429 $ 38,974 Non-U.S. equity securities 21,509 2,659 24,168 18,182 2,543 20,725 Fixed income securities 34,857 7,253 42,110 35,214 6,815 42,029 Cash and cash equivalents 2,924 — 2,924 1,723 — 1,723 Total $ 90,028 $ 16,708 $ 106,736 $ 87,664 $ 15,787 $ 103,451
Schedule of Future Benefit Payments	Benefit payments, which reflect future expected service as appropriate, are expected to be paid in each fiscal year as follows: (in thousands) 2021 $ 6,155 2022 6,436 2023 6,667 2024 6,879 2025 7,125 2026 to 2030 37,448

STOCK INCENTIVE PLAN (Tables)

STOCK INCENTIVE PLAN (Tables)	12 Months Ended
	Sep. 30, 2020
Share-based Payment Arrangement [Abstract]
Schedule of Assumptions	The assumptions used in the Black-Scholes option pricing model to value the options granted were as follows: Fiscal Year Ended September 30, 2020 September 30, 2019 September 30, 2018 Expected dividend yield — % — % — % Expected volatility 36 % 45 % 40 % Range of risk-free interest rates 1.67 % 2.94% - 3.00% 2.16% - 2.38% Range of expected option lives 6 years 6 to 10 years 1 to 6 years fiscal year ended September 30, 2020 were as follows: September 30, 2020 September 30, 2019 September 30, 2018 Expected dividend yield — % — % — % Range of expected volatility 21.45-53.18 20.69 - 74.26 17.21 - 79.15 Risk free interest rates 1.59 % 2.84 % 1.83% - 2.54% Expected life 3 years 3 years 1 -3 years Fair value $ 44.22 18.84 $ 13.90 - 26.68
Schedule of Stock Option Activity	Stock option activity for the period September 30, 2017 to September 30, 2020 was as follows: Shares Weighted-Average Exercise Price Weighted-Average Grant Date Fair Value Aggregate Intrinsic Value (in thousands) Weighted-Average Remaining Contractual Term (in years) Outstanding as of September 30, 2017 5,158 $ 8.30 Granted 185 20.01 8.33 Exercised (2,728 ) 7.55 $ 42,512 Forfeited (10 ) 8.28 Outstanding as of September 30, 2018 2,605 9.91 Granted 342 23.45 11.35 Exercised (985 ) 8.62 $ 16,453 Forfeited (126 ) 12.64 Outstanding as of September 30, 2019 1,836 12.94 $ 31,957 Granted 67 37.24 $ 14.05 Exercised (508 ) 8.55 $ 14,548 Forfeited (20 ) 19.13 Outstanding as of September 30, 2020 1,375 15.66 11,443 5.43 Exercisable as of September 30, 2020 1,012 $ 11.99 $ 11,013 4.48
Schedule of Nonvested Share Activity	Changes to the Company's nonvested RSU awards for the year ended September 30, 2020 were as follows: Shares Weighted-average grant-date fair value Nonvested as of September 30, 2017 435 $ 19.65 Granted 334 20.86 Vested (90 ) 21.95 Forfeited (29 ) 20.57 Nonvested as of September 30, 2018 650 $ 19.91 Granted 361 19.23 Vested (195 ) 21.51 Forfeited (142 ) 18.52 Nonvested as of September 30, 2019 674 19.37 Granted 166 37.24 Vested (403 ) 19.43 Forfeited (32 ) 22.29 Nonvested as of September 30, 2020 405 $ 26.44 Changes to the Company's non-vested PSU awards for the year ended September 30, 2020 were as follows: Shares Weighted-average grant-date fair value Nonvested as of September 30, 2017 181 $ 23.87 Granted 215 22.01 Forfeited (89 ) 28.53 Nonvested as of September 30, 2018 307 $ 21.22 Granted 242 18.17 Forfeited (94 ) 20.55 Nonvested as of September 30, 2019 455 $ 21.11 Granted 115 40.43 Vested (162 ) 25.21 Adjustment for achieved performance upon issuance 31 21.68 Forfeited (14 ) 22.82 Nonvested as of September 30, 2020 425 $ 25.15

RESTRUCTURING CHARGES (Tables)

RESTRUCTURING CHARGES (Tables)	12 Months Ended
	Sep. 30, 2020
Restructuring and Related Activities [Abstract]
Schedule of Restructuring Reserves	Restructuring reserves are included as a component of other current liabilities . Electrical Raceway Mechanical Products & Solutions Other/Corporate (in thousands) Severance (a) Other (b) Severance (a) Other (a) Severance (a) Total Balance as of September 30, 2017 $ 449 $ — $ 278 $ 10 $ — $ 737 Charges 536 1,130 97 179 98 2,040 Utilization (787 ) (820 ) (178 ) (160 ) (98 ) (2,043 ) Reversals — — (191 ) — — (191 ) Exchange rate effects 14 — (6 ) — — 8 Balance as of September 30, 2018 $ 212 $ 310 $ — $ 29 $ — $ 551 Charges 1,047 2,544 213 — — 3,804 Utilization (867 ) (2,854 ) (68 ) (29 ) — (3,818 ) Balance as of September 30, 2019 $ 392 $ — $ 145 $ — $ — $ 537 Charges $ 1,339 $ 1,892 $ 416 — 68 3,715 Utilization $ (1,175 ) $ (1,892 ) $ (429 ) — (52 ) (3,548 ) Reversals $ (369 ) $ — $ (60 ) — (2 ) (431 ) Exchange rate effects $ 18 $ — $ — — — 18 Balance as of September 30, 2020 $ 205 $ — $ 72 $ — $ 14 $ 291 (a) Fiscal 2020 charges are primarily related to a reduction in force implemented as part of adjusting to the macroeconomic impacts related to the outbreak of the novel coronavirus (COVID-19). Fiscal 2019 and 2018 charges are primarily related to Atkore's commitment to close and consolidate certain facilities as part of its continuing effort to realign its strategic focus. (b) Primarily related to Atkore's commitment to close and consolidate certain facilities as part of its continuing effort to realign its strategic focus. The Company expects to utilize all restructuring accruals as of September 30, 2020 within the next twelve months. The net restructuring charges included as a component of selling, general and administrative expense in the Company's consolidated statements of operations were as follows: Fiscal Year Ended (in thousands) September 30, 2020 September 30, 2019 September 30, 2018 Total restructuring charges, net $ 3,284 $ 3,804 $ 1,849

OTHER INCOME, NET (Tables)

OTHER INCOME, NET (Tables)	12 Months Ended
	Sep. 30, 2020
Other Income, Nonoperating [Abstract]
Other Income, Net	Other income, net consisted of the following: Fiscal Year Ended (in thousands) September 30, 2020 September 30, 2019 September 30, 2018 Gain on purchase of business $ — $ (7,384 ) $ — Gain on sale of a business — — (27,575 ) Undesignated foreign currency derivative instruments 1,269 (5,384 ) (121 ) Foreign exchange loss (gain) on intercompany loans (2,342 ) 2,975 1,500 Debt modification costs — — 892 Pension-related benefits (1,704 ) (1,606 ) (1,975 ) Other — (79 ) (69 ) Other income, net $ (2,777 ) $ (11,478 ) $ (27,348 )

INCOME TAXES (Tables)

INCOME TAXES (Tables)	12 Months Ended
	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Schedule of Components of Income (Loss) from Continuing Operations and Income Tax Expense	Significant components of income before income taxes and income tax expense for the fiscal years ended September 30, 2020 , September 30, 2019 and September 30, 2018 consisted of the following: (in thousands) September 30, 2020 September 30, 2019 September 30, 2018 Components of income before income taxes: United States $ 201,784 $ 170,780 $ 155,453 Non-U.S 214 13,889 10,899 Income before income taxes $ 201,998 $ 184,669 $ 166,352 Income tax expense: Current: United States: Federal $ 32,335 $ 31,431 $ 26,586 State 9,668 9,800 8,662 Non-U.S: 3,210 5,183 3,467 Current income tax expense $ 45,213 $ 46,414 $ 38,715 Deferred: United States: Federal $ 5,102 $ 1,123 $ (7,149 ) State 49 (340 ) (1,793 ) Non-U.S: (668 ) (1,579 ) (66 ) Deferred income (benefit) tax expense 4,483 (796 ) (9,008 ) Income tax expense $ 49,696 $ 45,618 $ 29,707
Schedule of Federal Income Tax Rate and Effective Income Tax Rate Reconciliation	Differences between the statutory federal income tax rate and effective income tax rate are summarized below: (in thousands) September 30, 2020 September 30, 2019 September 30, 2018 Statutory federal tax 21 % 21 % 25 % Adjustments to reconcile to the effective income tax rate: State income taxes 4 % 4 % 3 % Change in tax rates — % — % (3 )% Domestic manufacturing deduction — % — % (2 )% Indemnified uncertain tax benefits — % — % (2 )% Stock-based compensation (2 )% (1 )% (3 )% Other 2 % 1 % — % Effective income tax rate 25 % 25 % 18 %
Schedule of Components of Net Deferred Income Tax Assets	The components of the net deferred income tax assets are as follows: (in thousands) September 30, 2020 September 30, 2019 Deferred tax assets: Accrued liabilities and reserves $ 26,783 $ 32,664 Tax loss and credit carryforwards 15,637 13,296 Postretirement benefits 11,751 10,315 Inventory 5,240 8,813 Lease obligations 9,966 — Other 174 916 $ 69,551 $ 66,004 Deferred tax liabilities: Property, plant and equipment $ (17,648 ) $ (21,312 ) Intangible assets (44,774 ) (47,887 ) Loss on investment (5,153 ) (4,790 ) Right-of-use assets, net (9,546 ) — Other (4,065 ) (3,660 ) $ (81,186 ) $ (77,649 ) Net deferred tax liability before valuation allowance (11,635 ) (11,645 ) Valuation allowance (10,203 ) (7,764 ) Net deferred tax liability $ (21,838 ) $ (19,409 )
Schedule of Reconciliation of Unrecognized Tax Benefits, Excluding Interest and Penalties	A reconciliation of the beginning and ending amount of unrecognized tax benefit, excluding interest and penalties, is as follows: (in thousands) For the period from September 30, 2017 to September 30, 2020 Balance as of September 30, 2017 $ 3,578 Additions based on tax positions related to prior years 893 Settlements (3,107 ) Balance as of September 30, 2018 1,364 Additions based on tax positions related to prior years 244 Settlements (814 ) Balance as of September 30, 2019 794 Additions based on tax positions related to prior years 925 Additions based on tax positions related to current year 122 Settlements (227 ) Balance as of September 30, 2020 1,614
Schedule of Tax Years Subject to Examination	The following tax years remain subject to examination by the major tax jurisdictions as follows: Jurisdiction Years Open to Audit United States 2018, 2019 and 2020

EARNINGS PER SHARE (Tables)

EARNINGS PER SHARE (Tables)	12 Months Ended
	Sep. 30, 2020
Earnings Per Share [Abstract]
Schedule of Basic and Diluted Net Income Per Share	The following table sets forth the computation of basic and diluted earnings per share: Fiscal Year Ended (in thousands, except per share data) September 30, 2020 September 30, 2019 September 30, 2018 Numerator: Net income $ 152,302 $ 139,051 $ 136,645 Less: Undistributed earnings allocated to participating securities 3,356 3,726 2,456 Net income available to common shareholders $ 148,946 $ 135,325 $ 134,189 Denominator: Basic weighted average common shares outstanding 47,265 46,577 51,791 Effect of dilutive securities: Non-participating employee stock options (1) 779 1,200 2,298 Diluted weighted average common shares outstanding 48,044 47,777 54,089 Basic earnings per share $ 3.15 $ 2.91 $ 2.59 Diluted earnings per share $ 3.10 $ 2.83 $ 2.48 (1) Stock options to purchase approximately 0.3 million , 0.4 million , and 0.3 million shares of common stock were outstanding during the years ended September 30, 2020 , September 30, 2019 , and September 30, 2018 , respectively, but were not included in the calculation of diluted earnings per share as the impact of these would have been anti-dilutive.

ACCUMULATED OTHER COMPREHENSI_2

ACCUMULATED OTHER COMPREHENSIVE LOSS (Tables)	12 Months Ended
	Sep. 30, 2020
Equity [Abstract]
Schedule of Accumulated Other Comprehensive Income (Loss)	The following table presents the changes in accumulated other comprehensive loss by component, net of tax: (in thousands) Defined benefit pension items Currency translation adjustments Total Balance as of September 30, 2018 $ (6,048 ) $ (10,390 ) $ (16,438 ) Other comprehensive loss before reclassifications (15,513 ) (7,490 ) (23,003 ) Amounts reclassified from accumulated other comprehensive loss 76 — 76 Net current period other comprehensive loss (15,437 ) (7,490 ) (22,927 ) Reclassification of stranded tax benefits (1) (2,333 ) — (2,333 ) Balance as of September 30, 2019 $ (23,818 ) $ (17,880 ) (41,698 ) Other comprehensive income (loss) before reclassifications (7,604 ) 6,087 (1,517 ) Amounts reclassified from accumulated other comprehensive loss 661 — 661 Net current period other comprehensive income (loss) (6,943 ) 6,087 (856 ) Balance as of September 30, 2020 $ (30,761 ) $ (11,793 ) $ (42,554 ) (1) Due to the adoption of ASU 2018-02 stranded tax benefits related to its pension plans were reclassified from Accumulated other comprehensive loss and into Accumulated deficit
Reclassification out of Accumulated Other Comprehensive Income	The following is a summary of the amounts reclassified from accumulated other comprehensive loss to net income : Fiscal Year Ended (in thousands) September 30, 2020 September 30, 2019 September 30, 2018 Amortization of defined benefit pension items: Amortization of net loss (included within other income, net) $ 888 $ 101 $ 343 Tax expense (227 ) (25 ) (14 ) Net reclassifications for the period $ 661 $ 76 $ 329

INVENTORIES, NET (Tables)

INVENTORIES, NET (Tables)	12 Months Ended
	Sep. 30, 2020
Inventory Disclosure [Abstract]
Schedule of Inventory, Net	(in thousands) September 30, 2020 September 30, 2019 Purchased materials and manufactured parts, net $ 49,192 $ 52,742 Work in process, net 24,113 21,424 Finished goods, net 125,790 151,924 Inventories, net $ 199,095 $ 226,090

PROPERTY, PLANT AND EQUIPMENT (

PROPERTY, PLANT AND EQUIPMENT (Tables)	12 Months Ended
	Sep. 30, 2020
Property, Plant and Equipment [Abstract]
Schedule of Property, Plant and Equipment	Depreciation is calculated using the straight-line method over the estimated useful lives of the related assets as follows: Buildings 4 to 40 years Building improvements 3 to 20 years Machinery and equipment 1 to 20 years Leasehold improvements Lesser of remaining term of the lease or useful life Software 2 to 10 years As of September 30, 2020 and September 30, 2019 , property, plant and equipment at cost and accumulated depreciation were as follows: (in thousands) September 30, 2020 September 30, 2019 Land $ 20,460 $ 19,897 Buildings and related improvements 129,538 127,061 Machinery and equipment 332,260 318,421 Leasehold improvements 9,862 9,055 Software 27,028 24,835 Construction in progress 22,736 21,264 Property, plant and equipment 541,884 520,533 Accumulated depreciation (297,993 ) (259,830 ) Property, plant and equipment, net $ 243,891 $ 260,703

GOODWILL AND INTANGIBLE ASSETS

GOODWILL AND INTANGIBLE ASSETS (Tables)	12 Months Ended
	Sep. 30, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Carrying Amounts of Goodwill	C hanges in the carrying amount of goodwill are as follows: Segment (in thousands) Electrical Raceway Mechanical Products & Solutions Total Balance as of September 30, 2018 $ 133,566 $ 36,563 $ 170,129 Goodwill acquired during year 18,124 — 18,124 Exchange rate effects $ (2,022 ) — $ (2,022 ) Balance as of September 30, 2019 $ 149,668 $ 36,563 $ 186,231 Exchange rate effects 2,008 — 2,008 Balance as of September 30, 2020 $ 151,676 $ 36,563 $ 188,239
Schedule of Major Classes of Finite-Lived Intangible Assets	The following table provides the gross carrying value, accumulated amortization, and net carrying value for each major class of intangible assets: September 30, 2020 September 30, 2019 (in thousands) Weighted Average Useful Life (Years) Gross Carrying Value Accumulated Amortization Net Carrying Value Gross Carrying Value Accumulated Amortization Net Carrying Value Amortizable Intangible Assets: Customer relationships 11 $ 355,735 $ (202,677 ) $ 153,058 $ 353,256 $ (171,777 ) $ 181,479 Other 7 19,086 (9,675 ) 9,411 19,086 (7,761 ) 11,325 Total 374,821 (212,352 ) 162,469 372,342 (179,538 ) 192,804 Indefinite-lived Intangible Assets: Trade names 92,880 — 92,880 92,880 — 92,880 Total $ 467,701 $ (212,352 ) $ 255,349 $ 465,222 $ (179,538 ) $ 285,684
Schedule of Major Classes of Indefinite-Lived Intangible Assets	The following table provides the gross carrying value, accumulated amortization, and net carrying value for each major class of intangible assets: September 30, 2020 September 30, 2019 (in thousands) Weighted Average Useful Life (Years) Gross Carrying Value Accumulated Amortization Net Carrying Value Gross Carrying Value Accumulated Amortization Net Carrying Value Amortizable Intangible Assets: Customer relationships 11 $ 355,735 $ (202,677 ) $ 153,058 $ 353,256 $ (171,777 ) $ 181,479 Other 7 19,086 (9,675 ) 9,411 19,086 (7,761 ) 11,325 Total 374,821 (212,352 ) 162,469 372,342 (179,538 ) 192,804 Indefinite-lived Intangible Assets: Trade names 92,880 — 92,880 92,880 — 92,880 Total $ 467,701 $ (212,352 ) $ 255,349 $ 465,222 $ (179,538 ) $ 285,684
Schedule of Expected Amortization Expense for Intangible Assets	Expected amortization expense for intangible assets over the next five years and thereafter is as follows (in thousands) : 2021 $ 32,267 2022 30,859 2023 30,806 2024 26,287 2025 13,681 2026 and thereafter 28,569

DEBT (Tables)

DEBT (Tables)	12 Months Ended
	Sep. 30, 2020
Debt Disclosure [Abstract]
Schedule of Debt	Debt as of September 30, 2020 and September 30, 2019 was as follows: (in thousands) September 30, 2020 September 30, 2019 First Lien Term Loan Facility due December 22, 2023 811,540 $ 851,361 Deferred financing costs (7,804 ) (6,569 ) Other — 525 Total debt $ 803,736 $ 845,317 Less: Current portion — — Long-term debt $ 803,736 $ 845,317
Contractual Obligation, Fiscal Year Maturity Schedule	As of September 30, 2020 , future contractual maturities of long-term debt are as follows (in thousands) : 2021 $ — 2022 — 2023 — 2024 812,120 2025 — 2026 and thereafter $ —

FAIR VALUE MEASUREMENTS (Tables

FAIR VALUE MEASUREMENTS (Tables)	12 Months Ended
	Sep. 30, 2020
Fair Value Disclosures [Abstract]
Schedule of Assets and Liabilities Measured on Recurring Basis	The following table presents the recurring assets and liabilities measured at fair value as of September 30, 2020 and September 30, 2019 in accordance with the fair value hierarchy: September 30, 2020 September 30, 2019 (in thousands) Level 1 Level 2 Level 3 Level 1 Level 2 Level 3 Assets Cash equivalents $ 209,421 $ — $ — $ 72,132 $ — $ — Forward currency contracts — 2,209 — — 3,420 — Liabilities Forward currency contracts — $ 102 — — $ — —
Schedule of Assets and Liabilities Measured on Non-recurring Basis	The estimated fair value of financial instruments not carried at fair value in the consolidated balance sheets were as follows: September 30, 2020 September 30, 2019 (in thousands) Carrying Value Fair Value Carrying Value Fair Value First Lien Term Loan Facility due December 22, 2023 $ 812,120 $ 808,060 $ 852,120 $ 853,543 Total debt $ 812,120 $ 808,060 $ 852,120 $ 853,543

SEGMENT INFORMATION (Tables)

SEGMENT INFORMATION (Tables)	12 Months Ended
	Sep. 30, 2020
Segment Reporting [Abstract]
Schedule of Segment Information	Recorded amounts represent a proportional amount of the quantity of product produced for each segment. Certain assets, such as machinery and equipment and facilities, are not allocated to each segment despite serving both segments. These shared assets are reported within the MP&S segment. We allocate certain corporate operating expenses that directly benefit our operating segments, such as insurance and information technology, on a basis that reasonably approximates an estimate of the use of these services. Fiscal year ended September 30, 2020 September 30, 2019 September 30, 2018 (in thousands) External Net Sales Inter- segment Sales Adjusted EBITDA External Net Sales Inter- segment Sales Adjusted EBITDA External Net Sales Inter- segment Sales Adjusted EBITDA Electrical Raceway $ 1,328,721 $ 2,554 $ 299,485 $ 1,442,278 $ 1,215 $ 292,585 $ 1,365,067 $ 1,544 $ 255,260 MP&S 436,700 — $ 61,152 474,260 — $ 70,040 470,072 81 $ 51,339 Eliminations — (2,554 ) — (1,215 ) — (1,625 ) Consolidated operations $ 1,765,421 $ — $ 1,916,538 $ — $ 1,835,139 $ — Capital Expenditures Total Assets (in thousands) September 30, 2020 September 30, 2019 September 30, 2018 September 30, 2020 September 30, 2019 September 30, 2018 Electrical Raceway $ 17,667 $ 19,856 $ 16,389 $ 835,629 $ 871,771 $ 751,024 MP&S 13,870 13,934 14,267 215,350 252,708 291,164 Unallocated 2,233 1,070 7,845 507,546 312,516 281,872 Consolidated operations $ 33,770 $ 34,860 $ 38,501 $ 1,558,525 $ 1,436,995 $ 1,324,060 Presented below is a reconciliation of operating segment Adjusted EBITDA to Income before income taxes : Fiscal Year Ended (in thousands) September 30, 2020 September 30, 2019 September 30, 2018 Operating segment Adjusted EBITDA Electrical Raceway $ 299,485 $ 292,585 $ 255,260 MP&S 61,152 70,040 51,339 Total $ 360,637 $ 362,625 $ 306,599 Unallocated expenses (a) (34,002 ) (38,217 ) (35,050 ) Depreciation and amortization (74,470 ) (72,347 ) (66,890 ) Interest expense, net (40,062 ) (50,473 ) (40,694 ) Gain (loss) on extinguishment of debt (273 ) — — Restructuring charges (3,284 ) (3,804 ) (1,849 ) Stock-based compensation (13,064 ) (11,798 ) (14,664 ) Certain legal matters — — 4,833 Transaction costs (196 ) (1,200 ) (9,314 ) Gain on purchase of business — 7,384 — Gain on sale of a business — — 27,575 Other (b) 6,712 (7,501 ) (4,194 ) Income before income taxes $ 201,998 $ 184,669 $ 166,352 (a) Represents unallocated selling, general and administrative activities and associated expenses including, in part, executive, legal, finance, human resources, information technology, business development and communications, as well as certain costs and earnings of employee-related benefits plans, such as stock-based compensation and a portion of self-insured medical costs. (b) Represents other items, such as inventory reserves and adjustments, loss on disposal of property, plant and equipment, insurance recovery related to damages of property, plant and equipment, release of indemnified uncertain tax positions and realized or unrealized gain (loss) on foreign currency impacts of intercompany loans and related forward currency derivatives.
Schedule of Long-Lived Assets and Net Sales by Geography	The Company's long-lived assets and net sales by geography were as follows: Long-lived assets Net sales (in thousands) September 30, 2020 September 30, 2019 September 30, 2018 September 30, 2020 September 30, 2019 September 30, 2018 United States $ 203,694 $ 219,614 $ 201,101 $ 1,563,258 $ 1,689,194 $ 1,651,636 Other Americas 146 147 138 26,421 33,485 43,013 Europe 41,283 43,207 11,090 132,299 142,279 90,915 Asia-Pacific 1,759 1,998 2,386 43,443 51,580 49,575 Total $ 246,882 $ 264,966 $ 214,715 $ 1,765,421 $ 1,916,538 $ 1,835,139
Schedule of Net Sales From External Customers by Product Category	The table below shows the amount of net sales from external customers for each of the Company's product categories which accounted for 10% or more of consolidated net sales in any of the last three fiscal years: Fiscal Year Ended (in thousands) September 30, 2020 September 30, 2019 September 30, 2018 Metal Electrical Conduit and Fittings $ 484,476 $ 546,533 $ 517,935 Armored Cable and Fittings 306,460 360,494 336,388 PVC Electrical Conduit & Fittings 308,561 292,243 311,811 Cable Tray & Cable Ladders 193,306 199,303 145,795 Other raceway products 35,918 43,705 53,138 Electrical Raceway 1,328,721 1,442,278 1,365,067 Mechanical Pipe 244,902 259,613 253,381 Other MP&S products 191,798 214,647 216,691 MP&S 436,700 474,260 470,072 Net sales $ 1,765,421 $ 1,916,538 $ 1,835,139

QUARTERLY OPERATING RESULTS (_2

QUARTERLY OPERATING RESULTS (UNAUDITED) (Tables)	12 Months Ended
	Sep. 30, 2020
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Operating Results	The following table presents unaudited selected quarterly financial data for the years ended September 30, 2020 and September 30, 2019 . The operating results for any quarter are not necessarily indicative of the results of any future period. Three Months Ended (in thousands, except per share data) December 27, 2019 March 27, 2020 June 26, 2020 September 30, 2020 December 29, 2018 March 30, 2019 June 29, 2019 September 30, 2019 (1) Net sales $ 447,448 $ 455,654 $ 384,899 $ 477,420 $ 452,028 $ 469,309 $ 493,491 $ 501,710 Gross profit 116,844 131,603 95,813 147,054 110,256 117,088 126,134 143,722 Net income 34,790 39,193 24,078 54,241 26,949 29,555 36,550 45,997 Net income per share Basic (2) $ 0.72 $ 0.81 $ 0.50 $ 1.12 $ 0.56 $ 0.62 $ 0.77 $ 0.96 Diluted (2) 0.71 0.80 0.49 1.11 0.54 0.61 0.75 0.94 (1) Includes a $7,384 gain on the acquisition of Cor-Tek. See Note 3, "Acquisitions" for additional information. (2) The sum of the quarters may not equal the total of the respective year's earnings per share due to changes in the weighted average shares outstanding throughout the year.

BASIS OF PRESENTATION AND SUM_4

BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Share Repurchase Program (Details)	Sep. 30, 2020USD ($)vote	Feb. 05, 2019USD ($)	Dec. 28, 2018USD ($)	Aug. 31, 2017USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of vote per share, common stock | vote	1
Stock repurchase program, authorized amount		$ 50,000,000		$ 75,000,000
Remaining authorized repurchase amount	$ 35,000,000		$ 0

BASIS OF PRESENTATION AND SUM_5

BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Revenue Recognition (Details)	12 Months Ended
	Sep. 30, 2020
Minimum
Disaggregation of Revenue [Line Items]
Performance obligation and payment period	30 days
Maximum
Disaggregation of Revenue [Line Items]
Performance obligation and payment period	60 days

BASIS OF PRESENTATION AND SUM_6

BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Property, Plant and Equipment (Details)	12 Months Ended
	Sep. 30, 2020
Minimum | Buildings
Property, Plant and Equipment [Line Items]
Estimated useful lives (years)	4 years
Minimum | Building improvements
Property, Plant and Equipment [Line Items]
Estimated useful lives (years)	3 years
Minimum | Machinery and equipment
Property, Plant and Equipment [Line Items]
Estimated useful lives (years)	1 year
Minimum | Software
Property, Plant and Equipment [Line Items]
Estimated useful lives (years)	2 years
Maximum | Buildings
Property, Plant and Equipment [Line Items]
Estimated useful lives (years)	40 years
Maximum | Building improvements
Property, Plant and Equipment [Line Items]
Estimated useful lives (years)	20 years
Maximum | Machinery and equipment
Property, Plant and Equipment [Line Items]
Estimated useful lives (years)	20 years
Maximum | Software
Property, Plant and Equipment [Line Items]
Estimated useful lives (years)	10 years

BASIS OF PRESENTATION AND SUM_7

BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Goodwill and Indefinite-Lived Intangible Asset Impairments (Details) - USD ($)	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Finite-Lived Intangible Assets [Line Items]
Goodwill impairment	$ 0	$ 0	$ 0
Impairment indefinite-lived asset	$ 0	$ 0	$ 0
Customer relationships | Maximum
Finite-Lived Intangible Assets [Line Items]
Useful life (years)	14 years
Customer relationships | Minimum
Finite-Lived Intangible Assets [Line Items]
Useful life (years)	6 years
Other intangible assets | Maximum
Finite-Lived Intangible Assets [Line Items]
Useful life (years)	20 years
Other intangible assets | Minimum
Finite-Lived Intangible Assets [Line Items]
Useful life (years)	1 year

LEASES - Additional Information

LEASES - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Oct. 01, 2019	Sep. 30, 2018	Sep. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201602Member	us-gaap:AccountingStandardsUpdate201602Member
Operating lease, right-of-use asset	$ 37,666
Operating lease, liability	38,710
Cumulative effect adjustment as a result of adoption	(378,410)	$ (232,936)		$ (122,059)	$ (360,871)
Lease, cost	17,009
Total rental expense on all operating leases	17,077	17,403
Cost of sales
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Lease, cost	14,295
Selling, General and Administrative Expenses
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Lease, cost	2,714
Accumulated Deficit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment as a result of adoption	$ 64,154	200,396		$ 317,373	$ 42,433
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, right-of-use asset			$ 45,519
Operating lease, liability			46,941
Cumulative effect adjustment as a result of adoption		1,053
Cumulative Effect, Period of Adoption, Adjustment | Accumulated Deficit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment as a result of adoption		$ 1,053	$ 1,053

LEASES - Assets and Liabilities

LEASES - Assets and Liabilities (Details) $ in Thousands	Sep. 30, 2020USD ($)
Operating And Finance Lease, Right-of-Use Assets [Abstract]
Operating lease assets	$ 37,666
Finance lease assets	2,999
Total right-of-use assets, gross	40,665
Less: accumulated amortization	(1,973)
Right-of-use assets, net	38,692
Current liabilities:
Current portion of operating lease liabilities	15,356
Current portion of finance lease liabilities	430
Lease obligations	15,786
Noncurrent liabilities:
Operating lease liabilities	23,354
Finance lease liabilities	789
Long-term lease obligations	24,143
Total lease obligations	$ 39,929
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	atkr:OperatingAndFinanceLeaseLiabilityCurrent
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	atkr:OperatingAndFinanceLeaseLiabilityNoncurrent

LEASES - Lease Cost (Details)

LEASES - Lease Cost (Details) $ in Thousands	12 Months Ended
	Sep. 30, 2020USD ($)
Leases [Abstract]
Amortization of right-of-use assets	$ 14,803
Interest on lease liabilities	41
Variable lease costs	714
Short term lease costs	1,451
Total lease costs	$ 17,009

LEASES - Maturity of Lease Liab

LEASES - Maturity of Lease Liabilities (Details) $ in Thousands	Sep. 30, 2020USD ($)
Financing Leases
2021	$ 422
2022	406
2023	332
2024	89
2025	3
2026 and after	0
Total lease payments	1,252
Less: Interest	(33)
Present value of lease liabilities	1,219
Operating Leases
2021	12,902
2022	10,097
2023	7,228
2024	5,789
2025	3,451
2026 and after	3,535
Total lease payments	43,002
Less: Interest	(4,292)
Present value of lease liabilities	$ 38,710

LEASES - Schedule of Minimum Fu

LEASES - Schedule of Minimum Future Operating Lease Payments (Details) $ in Thousands	Sep. 30, 2020USD ($)
Leases [Abstract]
2020	$ 13,526
2021	11,592
2022	8,666
2023	6,362
2024	5,097
2025 and thereafter	6,938
Total	$ 52,181

LEASES - Lease Term and Discoun

LEASES - Lease Term and Discount Rate (Details)	Sep. 30, 2020
Weighted-average remaining lease term (years)
Operating leases	4 years 4 months 24 days
Finance leases	3 years 2 months 12 days
Weighted-average discount rate
Operating leases	3.60%
Finance leases	3.10%

LEASES - Cash Flow Information

LEASES - Cash Flow Information (Details) $ in Thousands	12 Months Ended
	Sep. 30, 2020USD ($)
Cash paid for amounts included in the measurement of lease liabilities
Operating cash flows from operating leases	$ 12,939
Operating cash flows from finance leases	26
Financing cash flows from finance leases	$ 500

ACQUISITIONS - Narrative (Detai

ACQUISITIONS - Narrative (Details) $ in Thousands	Aug. 21, 2019USD ($)	Jun. 03, 2019USD ($)	Oct. 01, 2018USD ($)country	Jan. 08, 2018USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2019USD ($)	Sep. 30, 2018USD ($)
Business Acquisition [Line Items]
Bargain purchase gain						$ 0	$ 7,384	$ 0
Goodwill					$ 188,239	$ 188,239	186,231	$ 170,129
Amortization period for tax-deductible goodwill (years)						15 years
Cor-Tek
Business Acquisition [Line Items]
Cash consideration, net of cash acquired	$ 14,835
Bargain purchase gain	$ 7,384				$ 7,384
U.S. Tray
Business Acquisition [Line Items]
Cash consideration, net of cash acquired		$ 25,507
Goodwill		7,295
Intangible assets		14,800
Working capital adjustment		$ 3,412
Vergokan
Business Acquisition [Line Items]
Cash consideration, net of cash acquired			$ 57,899
Goodwill							9,629
Intangible assets							$ 12,621
Number of countries in which entity operates | country			45
Communications Integrators, Inc
Business Acquisition [Line Items]
Cash consideration, net of cash acquired				$ 3,997
Selling, General and Administrative Expenses
Business Acquisition [Line Items]
Acquisition-related expenses						$ 1,140

ACQUISITIONS - Summary of Level

ACQUISITIONS - Summary of Level 3 Fair Values Assigned to Net Assets Acquired and Liabilities Assumed As of Acquisition Date (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2018
Fair value of assets acquired and liabilities assumed:
Excess purchase price attributed to goodwill acquired	$ 186,231	$ 188,239	$ 170,129
Vergokan
Fair value of consideration transferred:
Cash consideration	58,728
Other liability consideration	0
Total consideration transferred	58,728
Fair value of assets acquired and liabilities assumed:
Cash	829
Accounts receivable	8,761
Inventories	11,434
Intangible assets	12,621
Fixed assets	32,490
Accounts payable	(18,716)
Gain on purchase of business	0
Other	1,680
Net assets acquired	49,099
Excess purchase price attributed to goodwill acquired	9,629
Other
Fair value of consideration transferred:
Cash consideration	41,641
Other liability consideration	1,400
Total consideration transferred	43,041
Fair value of assets acquired and liabilities assumed:
Cash	1,541
Accounts receivable	8,217
Inventories	7,494
Intangible assets	16,400
Fixed assets	19,298
Accounts payable	(7,608)
Gain on purchase of business	(7,384)
Other	(3,412)
Net assets acquired	34,546
Excess purchase price attributed to goodwill acquired	8,495
Acquisitions For Fiscal Year 2019
Fair value of consideration transferred:
Cash consideration	100,369
Other liability consideration	1,400
Total consideration transferred	101,769
Fair value of assets acquired and liabilities assumed:
Cash	2,370
Accounts receivable	16,978
Inventories	18,928
Intangible assets	29,021
Fixed assets	51,788
Accounts payable	(26,324)
Gain on purchase of business	(7,384)
Other	(1,732)
Net assets acquired	83,645
Excess purchase price attributed to goodwill acquired	$ 18,124

ACQUISITIONS - Summary of Fair

ACQUISITIONS - Summary of Fair Value as of Acquisition Date (Details) $ in Thousands	12 Months Ended
	Sep. 30, 2019USD ($)
Vergokan
Business Acquisition [Line Items]
Intangible assets	$ 12,621
Other
Business Acquisition [Line Items]
Intangible assets	16,400
Customer relationships | Vergokan
Business Acquisition [Line Items]
Intangible assets	$ 10,535
Weighted Average Useful Life (Years)	12 years
Customer relationships | Other
Business Acquisition [Line Items]
Intangible assets	$ 15,400
Weighted Average Useful Life (Years)	10 years
Other | Vergokan
Business Acquisition [Line Items]
Intangible assets	$ 2,086
Weighted Average Useful Life (Years)	9 years
Other | Other
Business Acquisition [Line Items]
Intangible assets	$ 1,000
Weighted Average Useful Life (Years)	9 years

DIVESTITURES (Details)

DIVESTITURES (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash consideration	$ 0	$ 0	$ 42,631
FlexHead Industries, Inc and SprinkFLEX, LLC | Disposed of by Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash consideration			42,631
Net assets divested			15,056
Gain on sale of a business			27,575
Mechanical Products & Solutions
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Goodwill divested during year			$ 2,626

POSTRETIREMENT BENEFITS - Net P

POSTRETIREMENT BENEFITS - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Weighted-average assumptions used to determine net periodic pension cost during the period:
Discount rate	3.10%	4.20%	3.70%
Expected return on plan assets	6.30%	6.30%	6.40%
Pension Plan
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Interest cost	$ 3,739	$ 4,665	$ 4,098
Expected return on plan assets	(6,331)	(6,372)	(6,416)
Amortization of actuarial loss	888	101	343
Net periodic (benefit) cost	$ (1,704)	$ (1,606)	$ (1,975)

POSTRETIREMENT BENEFITS - Chang

POSTRETIREMENT BENEFITS - Change in Benefit Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Amounts recognized in the consolidated balance sheets consist of:
Pension liabilities	$ (40,023)	$ (34,509)
Pension Plan
Change in benefit obligations:
Beginning balance, benefit obligations	137,960	120,957
Interest cost	3,739	4,665	$ 4,098
Actuarial loss	10,476	17,463
Benefits and administrative expenses paid	(5,416)	(5,125)
Ending balance, benefit obligations	146,759	137,960	120,957
Change in plan assets:
Beginning balance, fair value of plan assets	103,451	104,454
Actual return on plan assets	6,587	3,043
Employer contributions	2,114	1,079
Benefits and administrative expenses paid	(5,416)	(5,125)
Ending balance, fair value of plan assets	106,736	103,451	$ 104,454
Amounts recognized in the consolidated balance sheets consist of:
Pension liabilities	(40,023)	(34,509)
Net amount recognized	(40,023)	(34,509)
Amounts recognized in accumulated other comprehensive loss (before income taxes) consist of:
Net actuarial loss	(42,367)	(33,035)
Total loss recognized	$ (42,367)	$ (33,035)
Weighted-average assumptions used to determine pension benefit obligations at year end:
Discount rate	2.50%	3.10%

POSTRETIREMENT BENEFITS - Benef

POSTRETIREMENT BENEFITS - Benefit Obligation In Excess Of Plan Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan, Pension Plan with Project Benefit Obligation in Excess of Plan Assets [Abstract]
Accumulated benefit obligation	$ 146,759	$ 137,960
Fair value of plan assets	106,736	103,451
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets [Abstract]
Projected benefit obligation	146,759	137,960
Fair value of plan assets	$ 106,736	$ 103,451

POSTRETIREMENT BENEFITS - Narra

POSTRETIREMENT BENEFITS - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plans expense	$ 3,718	$ 3,572	$ 3,600
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	2,114	1,079
Estimated employer contributions in next fiscal year	1,855
Multiemployer Plan [Line Items]
Liability	$ 5,345	$ 5,658
Pension Plan | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Policy target allocation (as a percentage)	60.00%
Pension Plan | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Policy target allocation (as a percentage)	40.00%

POSTRETIREMENT BENEFITS - Weigh

POSTRETIREMENT BENEFITS - Weighted-Average Asset Allocations (Details) - Pension Plan	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Plan asset allocations	100.00%	100.00%
Equity securities
Defined Benefit Plan Disclosure [Line Items]
Plan asset allocations	58.00%	58.00%
Debt securities
Defined Benefit Plan Disclosure [Line Items]
Plan asset allocations	39.00%	40.00%
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Plan asset allocations	3.00%	2.00%

POSTRETIREMENT BENEFITS - Asset

POSTRETIREMENT BENEFITS - Asset Allocations by Fair Value Hierarchy (Details) - Pension Plan - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 106,736	$ 103,451	$ 104,454
United States equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	37,534	38,974
Non-U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	24,168	20,725
Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	42,110	42,029
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,924	1,723
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	90,028	87,664
Level 1 | United States equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	30,738	32,545
Level 1 | Non-U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	21,509	18,182
Level 1 | Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	34,857	35,214
Level 1 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,924	1,723
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	16,708	15,787
Level 2 | United States equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,796	6,429
Level 2 | Non-U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,659	2,543
Level 2 | Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7,253	6,815
Level 2 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0	$ 0

POSTRETIREMENT BENEFITS - Futur

POSTRETIREMENT BENEFITS - Future Expected Benefit Payments (Details) $ in Thousands	Sep. 30, 2020USD ($)
Retirement Benefits [Abstract]
2021	$ 6,155
2022	6,436
2023	6,667
2024	6,879
2025	7,125
2026 to 2030	$ 37,448

STOCK INCENTIVE PLAN - Narrativ

STOCK INCENTIVE PLAN - Narrative (Details) - USD ($) $ in Thousands, shares in Millions	Jun. 10, 2016	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Nov. 21, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense		$ 13,064	$ 11,798	$ 14,664
Employee service share-based compensation, tax benefit from compensation expense		$ 3,014	$ 2,949	3,681
Omnibus Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance (shares)					1.1
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average period (years)		1 year 7 months 6 days
Stock Appreciation Rights (SARs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Life of award	10 years
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average period (years)		1 year 7 months 20 days
Additional expense				$ 500

STOCK INCENTIVE PLAN - Stock Op

STOCK INCENTIVE PLAN - Stock Options Assumptions (Details) - Stock Options	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	0.00%	0.00%	0.00%
Expected volatility	36.00%	45.00%	40.00%
Range of risk-free interest rates	1.67%
Range of risk free interest rates, minimum		2.94%	2.16%
Range of risk free interest rates, maximum		3.00%	2.38%
Range of expected option lives	6 years
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Range of expected option lives		6 years	1 year
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Range of expected option lives		10 years	6 years

STOCK INCENTIVE PLAN - Stock _2

STOCK INCENTIVE PLAN - Stock Options Activity (Details) - Stock Options - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Shares (in thousands)
Outstanding, beginning balance (shares)	1,836	2,605	5,158
Granted (shares)	67	342	185
Exercised (shares)	(508)	(985)	(2,728)
Forfeited (shares)	(20)	(126)	(10)
Outstanding, ending balance (shares)	1,375	1,836	2,605
Vested (shares)	1,012
Weighted-Average Exercise Price
Outstanding, beginning balance (in dollars per share)	$ 12.94	$ 9.91	$ 8.30
Granted (in dollars per share)	37.24	23.45	20.01
Exercised (in dollars per share)	8.55	8.62	7.55
Forfeited (in dollars per share)	19.13	12.64	8.28
Outstanding, ending balance (in dollars per share)	15.66	12.94	9.91
Vested (in dollars per share)	11.99
Stock Options, Additional Disclosures
Weighted average grant date fair value of options granted (in dollars per share)	$ 14.05	$ 11.35	$ 8.33
Exercised, aggregate intrinsic value	$ 14,548	$ 16,453	$ 42,512
Outstanding, aggregate intrinsic value	11,443	$ 31,957
Vested, aggregate intrinsic value	$ 11,013
Outstanding, weighted average remaining contractual term (in years)	5 years 5 months 4 days
Vested, weighted average remaining contractual term (in years)	4 years 5 months 23 days

STOCK INCENTIVE PLAN - Stock _3

STOCK INCENTIVE PLAN - Stock Options Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of shares vested	$ 1,894
Proceeds from stock options exercised, gross	4,347
Issuance of common stock, net of taxes withheld		$ 7,374
Issuance of common stock, net of taxes withheld	(2,972)	7,374	$ 20,110
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense	$ 1,289
Weighted-average period (years)	1 year 2 months 12 days
Fair value of shares vested		3,038	9,757
Tax benefit for tax deductions from stock options exercised	$ 3,681	$ 4,031	$ 11,478

STOCK INCENTIVE PLAN - Restrict

STOCK INCENTIVE PLAN - Restricted Stock Narrative (Details) - Restricted Stock Units (RSUs) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (years)	3 years
Unrecognized compensation expense	$ 5,185
Weighted-average period (years)	1 year 7 months 20 days
Fair value of shares vested in period	$ 14,513	$ 4,271	$ 1,969

STOCK INCENTIVE PLAN - Schedule

STOCK INCENTIVE PLAN - Schedule of Nonvested Restricted Stock (Details) - Restricted Stock Units (RSUs) - $ / shares shares in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Nonvested, beginning balance (shares)	674	650	435
Nonvested, granted (shares)	166	361	334
Nonvested, vested (shares)	(403)	(195)	(90)
Nonvested, forfeited (shares)	(32)	(142)	(29)
Nonvested, ending balance (shares)	405	674	650
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Nonvested, beginning of year (in dollars per share)	$ 19.37	$ 19.91	$ 19.65
Nonvested, granted (in dollars per share)	37.24	19.23	20.86
Nonvested, vested (in dollars per share)	19.43	21.51	21.95
Nonvested, forfeited (in dollars per share)	22.29	18.52	20.57
Nonvested, beginning of year (in dollars per share)	$ 26.44	$ 19.37	$ 19.91

STOCK INCENTIVE PLAN - Performa

STOCK INCENTIVE PLAN - Performance Share Units Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	Nov. 30, 2016	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Closing stock price (in dollars per share)		$ 37.24	$ 17.90	$ 20.01
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance condition, percentage of the award and a market condition	70.00%
Performance condition, peer group, remaining percentage	30.00%
Award vesting period (years)	3 years
Unrecognized compensation expense		$ 2,783
Weighted-average period (years)		1 year 7 months 6 days
Nonvested, vested (shares)		(162,000)	0	0

STOCK INCENTIVE PLAN - Perfor_2

STOCK INCENTIVE PLAN - Performance Share Assumptions (Details) - Performance Shares - $ / shares	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	0.00%	0.00%	0.00%
Expected volatility, minimum	21.45%	20.69%	17.21%
Expected volatility, maximum	53.18%	74.26%	79.15%
Range of risk-free interest rates	1.59%	2.84%
Range of risk-free interest rates, minimum (%)			1.83%
Range of risk-free interest rates, maximum (%)			2.54%
Expected life	3 years	3 years
Fair value (in dollars per share)	$ 44.22	$ 18.84
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life			1 year
Fair value (in dollars per share)			$ 13.90
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life			3 years
Fair value (in dollars per share)			$ 26.68

STOCK INCENTIVE PLAN - Schedu_2

STOCK INCENTIVE PLAN - Schedule of Nonvested Performance Shares (Details) - Performance Shares - $ / shares	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Nonvested, beginning balance (shares)	455,000	307,000	181,000
Shares issued (shares)	115,000	242,000	215,000
Nonvested, vested (shares)	(162,000)	0	0
Adjustment for achieved performance (shares)	31,000
Nonvested, forfeited (shares)	(14,000)	(94,000)	(89,000)
Nonvested, ending balance (shares)	425,000	455,000	307,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Nonvested, beginning of year (in dollars per share)	$ 21.11	$ 21.22	$ 23.87
Grant date fair value (in dollars per share)	40.43	18.17	22.01
Nonvested, vested (in dollars per share)	25.21
Adjustment for achieved performance (in dollars per share)	21.68
Nonvested, forfeited (in dollars per share)	22.82	20.55	28.53
Nonvested, beginning of year (in dollars per share)	$ 25.15	$ 21.11	$ 21.22

RESTRUCTURING CHARGES - Restruc

RESTRUCTURING CHARGES - Restructuring Reserves (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Restructuring Reserve [Roll Forward]
Beginning balance	$ 537	$ 551	$ 737
Charges	3,715	3,804	2,040
Utilization	(3,548)	(3,818)	(2,043)
Reversals	(431)		(191)
Exchange rate effects	18		8
Ending balance	291	537	551
Other/Corporate | Severance
Restructuring Reserve [Roll Forward]
Beginning balance	0	0	0
Charges	68	0	98
Utilization	(52)	0	(98)
Reversals	(2)		0
Exchange rate effects	0		0
Ending balance	14	0	0
Electrical Raceway | Operating Segments | Severance
Restructuring Reserve [Roll Forward]
Beginning balance	392	212	449
Charges	1,339	1,047	536
Utilization	(1,175)	(867)	(787)
Reversals	(369)		0
Exchange rate effects	18		14
Ending balance	205	392	212
Electrical Raceway | Operating Segments | Other
Restructuring Reserve [Roll Forward]
Beginning balance	0	310	0
Charges	1,892	2,544	1,130
Utilization	(1,892)	(2,854)	(820)
Reversals	0		0
Exchange rate effects	0		0
Ending balance	0	0	310
Mechanical Products & Solutions | Operating Segments | Severance
Restructuring Reserve [Roll Forward]
Beginning balance	145	0	278
Charges	416	213	97
Utilization	(429)	(68)	(178)
Reversals	(60)		(191)
Exchange rate effects	0		(6)
Ending balance	72	145	0
Mechanical Products & Solutions | Operating Segments | Other
Restructuring Reserve [Roll Forward]
Beginning balance	0	29	10
Charges	0	0	179
Utilization	0	(29)	(160)
Reversals	0		0
Exchange rate effects	0		0
Ending balance	$ 0	$ 0	$ 29

RESTRUCTURING CHARGES - Schedul

RESTRUCTURING CHARGES - Schedule of Restructuring Charges, Net (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Restructuring and Related Activities [Abstract]
Total restructuring charges, net	$ 3,284	$ 3,804	$ 1,849

OTHER INCOME, NET - Schedule of

OTHER INCOME, NET - Schedule of Other Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Other Income, Nonoperating [Abstract]
Gain on purchase of business	$ 0	$ (7,384)	$ 0
Gain on sale of a business	0	0	(27,575)
Undesignated foreign currency derivative instruments	1,269	(5,384)	(121)
Foreign exchange loss (gain) on intercompany loans	(2,342)	2,975	1,500
Debt modification costs	0	0	892
Pension-related benefits	(1,704)	(1,606)	(1,975)
Other	0	(79)	(69)
Other income, net	$ (2,777)	$ (11,478)	$ (27,348)

INCOME TAXES - Components of In

INCOME TAXES - Components of Income (Loss) from Continuing Operations and Income Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Components of income before income taxes:
United States	$ 201,784	$ 170,780	$ 155,453
Non-U.S	214	13,889	10,899
Income before income taxes	201,998	184,669	166,352
United States:
Federal	32,335	31,431	26,586
State	9,668	9,800	8,662
Non-U.S:	3,210	5,183	3,467
Current income tax expense	45,213	46,414	38,715
United States:
Federal	5,102	1,123	(7,149)
State	49	(340)	(1,793)
Non-U.S:	(668)	(1,579)	(66)
Deferred income (benefit) tax expense	4,483	(796)	(9,008)
Income tax expense	$ 49,696	$ 45,618	$ 29,707

INCOME TAXES - Differences Betw

INCOME TAXES - Differences Between Federal Income Tax Rate and Effective Income Tax Rate (Details)	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Statutory federal tax	21.00%	21.00%	25.00%
Adjustments to reconcile to the effective income tax rate:
State income taxes	4.00%	4.00%	3.00%
Change in tax rates	0.00%	0.00%	(3.00%)
Domestic manufacturing deduction	0.00%	0.00%	(2.00%)
Indemnified uncertain tax benefits	0.00%	0.00%	(2.00%)
Stock-based compensation	(2.00%)	(1.00%)	(3.00%)
Other	2.00%	1.00%	0.00%
Effective income tax rate	25.00%	25.00%	18.00%

INCOME TAXES - Narrative (Detai

INCOME TAXES - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Operating Loss Carryforwards [Line Items]
State income tax expense	$ 7,690	$ 7,566	$ 5,675
Current year valuation allowance expense	2,099
Limitations on executive compensation	1,243
Effective income tax reconciliation, excess tax benefit stock option on exercise of stock options, amount	$ 4,203	1,537	4,091
Benefit from the remeasurement of the deferred tax liabilities as a result of the TCJA			5,466
GILTI provisions		953
GILTI withholding taxes		635
Effective income tax rate reconciliation, acquisition of business		1,551
Release of indemnification tax positions			3,977
Tax benefit for manufacturing deduction			3,309
Operating loss carryforwards expiration term (year)	5 years
Cumulative loss position (year)	3 years
Unrecognized tax benefits	$ 1,614	794	1,364	$ 3,578
Accrued interest and penalties	188	54
Unrecognized tax benefits, increase	1,047
Settlements	227	814	3,107
Additions based on tax positions related to prior years	925	244	893
Increase (Decrease) of accrued penalties and interest for uncertain tax positions	(134)	25	$ 3,139
Undistributed income	59,719
State
Operating Loss Carryforwards [Line Items]
Operating loss carrying forwards	44,169
Non-U.S.
Operating Loss Carryforwards [Line Items]
Operating loss carrying forwards	$ 52,092
Decrease of deferred tax assets, valuation allowance		$ 550

INCOME TAXES - Components of Ne

INCOME TAXES - Components of Net Deferred Income Tax Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Deferred tax assets:
Accrued liabilities and reserves	$ 26,783	$ 32,664
Tax loss and credit carryforwards	15,637	13,296
Postretirement benefits	11,751	10,315
Inventory	5,240	8,813
Lease obligations	9,966
Other	174	916
Deferred tax assets	69,551	66,004
Deferred tax liabilities:
Property, plant and equipment	(17,648)	(21,312)
Intangible assets	(44,774)	(47,887)
Loss on investment	(5,153)	(4,790)
Right-of-use assets, net	(9,546)
Other	(4,065)	(3,660)
Deferred tax liabilities	(81,186)	(77,649)
Net deferred tax liability before valuation allowance	(11,635)	(11,645)
Valuation allowance	(10,203)	(7,764)
Net deferred tax liability	$ (21,838)	$ (19,409)

INCOME TAXES - Unrecognized Tax

INCOME TAXES - Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance	$ 794	$ 1,364	$ 3,578
Additions based on tax positions related to prior years	925	244	893
Additions based on tax positions related to current year	122
Settlements	(227)	(814)	(3,107)
Ending balance	$ 1,614	$ 794	$ 1,364

EARNINGS PER SHARE - Schedule o

EARNINGS PER SHARE - Schedule of Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 26, 2020	Mar. 27, 2020	Dec. 27, 2019	Sep. 30, 2019	Jun. 28, 2019	Mar. 29, 2019	Dec. 28, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Numerator:
Net income	$ 54,241	$ 24,078	$ 39,193	$ 34,790	$ 45,997	$ 36,550	$ 29,555	$ 26,949	$ 152,302	$ 139,051	$ 136,645
Less: Undistributed earnings allocated to participating securities									3,356	3,726	2,456
Net income available to common shareholders									$ 148,946	$ 135,325	$ 134,189
Denominator:
Basic weighted average common shares outstanding (shares)									47,265	46,577	51,791
Effect of dilutive securities: non-participating employee stock options (shares)									779	1,200	2,298
Diluted weighted average common shares outstanding (shares)									48,044	47,777	54,089
Basic (in dollars per share)	$ 1.12	$ 0.50	$ 0.81	$ 0.72	$ 0.96	$ 0.77	$ 0.62	$ 0.56	$ 3.15	$ 2.91	$ 2.59
Diluted (in dollars per share)	$ 1.11	$ 0.49	$ 0.80	$ 0.71	$ 0.94	$ 0.75	$ 0.61	$ 0.54	$ 3.10	$ 2.83	$ 2.48
Common Stock
Denominator:
Antidilutive securities excluded from computation of earnings per share, amount									300	400	300

ACCUMULATED OTHER COMPREHENSI_3

ACCUMULATED OTHER COMPREHENSIVE LOSS - Schedule of Change in Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands		12 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		$ 232,936	$ 122,059	$ 360,871
Other comprehensive loss before reclassifications		(1,517)	(23,003)
Amounts reclassified from accumulated other comprehensive loss		661	76
Total other comprehensive (loss) income		(856)	(22,927)	1,544
Reclassification of stranded tax benefits	[1]		0
Ending balance		378,410	232,936	122,059
Defined benefit pension items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		(23,818)	(6,048)
Other comprehensive loss before reclassifications		(7,604)	(15,513)
Amounts reclassified from accumulated other comprehensive loss		661	76	329
Total other comprehensive (loss) income		(6,943)	(15,437)
Reclassification of stranded tax benefits			(2,333)
Ending balance		(30,761)	(23,818)	(6,048)
Accumulated Foreign Currency Adjustment Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		(17,880)	(10,390)
Other comprehensive loss before reclassifications		6,087	(7,490)
Amounts reclassified from accumulated other comprehensive loss		0	0
Total other comprehensive (loss) income		6,087	(7,490)
Reclassification of stranded tax benefits			0
Ending balance		(11,793)	(17,880)	(10,390)
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		(41,698)	(16,438)	(17,982)
Total other comprehensive (loss) income		(856)	(22,927)	1,544
Reclassification of stranded tax benefits	[1]		(2,333)
Ending balance		$ (42,554)	$ (41,698)	$ (16,438)
[1]	Due to the adoption of ASU 2018-02.

ACCUMULATED OTHER COMPREHENSI_4

ACCUMULATED OTHER COMPREHENSIVE LOSS - Summary of Amounts Reclassified (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net reclassifications for the period	$ 661	$ 76
Defined benefit pension items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of net loss (included within other income, net)	888	101	$ 343
Tax expense	(227)	(25)	(14)
Net reclassifications for the period	$ 661	$ 76	$ 329

INVENTORIES, NET - Narrative (D

INVENTORIES, NET - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Inventory Disclosure [Abstract]
Inventories at lower of LIFO cost or market (%)	73.00%	72.00%
Inventory, LIFO reserve	$ 4,418	$ (3,138)
Effect of LIFO inventory liquidation on operating income	1,837
Excess and obsolete inventory reserve	$ 14,533	$ 14,295

INVENTORIES, NET - Schedule of

INVENTORIES, NET - Schedule of Company Inventories (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Inventory Disclosure [Abstract]
Purchased materials and manufactured parts, net	$ 49,192	$ 52,742
Work in process, net	24,113	21,424
Finished goods, net	125,790	151,924
Inventories, net	$ 199,095	$ 226,090

PROPERTY, PLANT AND EQUIPMENT -

PROPERTY, PLANT AND EQUIPMENT - Schedule of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Property, Plant and Equipment [Line Items]
Property, plant and equipment	$ 541,884	$ 520,533
Accumulated depreciation	(297,993)	(259,830)
Property, plant and equipment, net	243,891	260,703
Land
Property, Plant and Equipment [Line Items]
Property, plant and equipment	20,460	19,897
Buildings and related improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment	129,538	127,061
Machinery and equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment	332,260	318,421
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment	9,862	9,055
Software
Property, Plant and Equipment [Line Items]
Property, plant and equipment	27,028	24,835
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment	$ 22,736	$ 21,264

PROPERTY, PLANT AND EQUIPMENT_2

PROPERTY, PLANT AND EQUIPMENT - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 42,208	$ 39,471	$ 34,786

GOODWILL AND INTANGIBLE ASSET_2

GOODWILL AND INTANGIBLE ASSETS - Goodwill (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Goodwill [Roll Forward]
Beginning balance	$ 186,231	$ 170,129
Goodwill acquired during year		18,124
Exchange rate effects	2,008	(2,022)
Ending balance	188,239	186,231
Electrical Raceway
Goodwill [Roll Forward]
Beginning balance	149,668	133,566
Goodwill acquired during year		18,124
Exchange rate effects	2,008	(2,022)
Ending balance	151,676	149,668
Mechanical Products & Solutions
Goodwill [Roll Forward]
Beginning balance	36,563	36,563
Goodwill acquired during year		0
Exchange rate effects	0	0
Ending balance	$ 36,563	$ 36,563

GOODWILL AND INTANGIBLE ASSET_3

GOODWILL AND INTANGIBLE ASSETS - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Indefinite-lived Intangible Assets [Line Items]
Amortization expense	$ 32,262	$ 32,876	$ 32,104
Electrical Raceway
Indefinite-lived Intangible Assets [Line Items]
Accumulated impairment loss	3,924	3,294
MP&S
Indefinite-lived Intangible Assets [Line Items]
Accumulated impairment loss	$ 43,000	$ 43,000

GOODWILL AND INTANGIBLE ASSET_4

GOODWILL AND INTANGIBLE ASSETS - Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Amortizable intangible assets:
Gross Carrying Value	$ 374,821	$ 372,342
Accumulated Amortization	(212,352)	(179,538)
Net Carrying Value	162,469	192,804
Intangible Assets, Net (Excluding Goodwill) [Abstract]
Gross Carrying Value	467,701	465,222
Net Carrying Value	255,349	285,684
Trade names
Indefinite-lived Intangible Assets [Line Items]
Gross carrying value/net carrying value	92,880	92,880
Customer relationships
Amortizable intangible assets:
Gross Carrying Value	355,735	353,256
Accumulated Amortization	(202,677)	(171,777)
Net Carrying Value	$ 153,058	181,479
Customer relationships | Weighted Average
Amortizable intangible assets:
Weighted Average Useful Life (Years)	11 years
Other
Amortizable intangible assets:
Gross Carrying Value	$ 19,086	19,086
Accumulated Amortization	(9,675)	(7,761)
Net Carrying Value	$ 9,411	$ 11,325
Other | Weighted Average
Amortizable intangible assets:
Weighted Average Useful Life (Years)	7 years

GOODWILL AND INTANGIBLE ASSET_5

GOODWILL AND INTANGIBLE ASSETS - Expected Amortization Expense (Details) $ in Thousands	Sep. 30, 2020USD ($)
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
2021	$ 32,267
2022	30,859
2023	30,806
2024	26,287
2025	13,681
2026 and thereafter	$ 28,569

DEBT - Schedule of Debt (Detail

DEBT - Schedule of Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Debt Instrument [Line Items]
Deferred financing costs	$ (7,804)	$ (6,569)
Total debt	803,736	845,317
Less: Current portion	0	0
Long-term debt	803,736	845,317
Secured Debt | First Lien Term Loan Facility due December 22, 2023
Debt Instrument [Line Items]
Carrying value	811,540	851,361
Other	$ 0	$ 525

DEBT - First Lien Loan - Narrat

DEBT - First Lien Loan - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Mar. 29, 2019
Line of Credit | Atkore International | ABL Credit Facility
Debt Instrument [Line Items]
Repayments of credit facility	$ 40,000	$ 40,000	$ 18,680

DEBT - Contractual Obligation (

DEBT - Contractual Obligation (Details) $ in Thousands	Sep. 30, 2020USD ($)
Debt Disclosure [Abstract]
2021	$ 0
2022	0
2023	0
2024	812,120
2025	0
2026 and thereafter	$ 0

DEBT - Term Loan Facilities - N

DEBT - Term Loan Facilities - Narrative (Details) - USD ($) shares in Millions	Feb. 02, 2018	Dec. 22, 2016	Apr. 09, 2014	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Debt Instrument [Line Items]
Repayments of secured debt				$ 40,000,000	$ 40,000,000	$ 1,217,000
Payments for repurchase of common stock				15,011,000	24,419,000	411,775,000
Debt modification costs				0	0	$ 892,000
Secured Debt | First Lien Term Loan Facility due December 22, 2023
Debt Instrument [Line Items]
Carrying value				$ 811,540,000	$ 851,361,000
Secured Debt | New First Lien Term Loan Facility
Debt Instrument [Line Items]
Carrying value	$ 425,000,000
Repurchase of stock (in shares)	17.2
Payments for repurchase of common stock	$ 375,000,000
Repayments of debt	42,000,000
Debt instrument, fee amount	$ 5,800,000
Secured Debt | New First Lien Term Loan Facility | Alternate Base Rate
Debt Instrument [Line Items]
Debt instrument, interest rate, stated percentage (%)	1.75%
Secured Debt | ABR Loans
Debt Instrument [Line Items]
Debt instrument, interest rate, stated percentage (%)	2.75%
Atkore International | Secured Debt
Debt Instrument [Line Items]
Repayments of secured debt		$ 155,000,000
Atkore International | Secured Debt | First Lien Term Loan Facility due December 22, 2023
Debt Instrument [Line Items]
Credit agreement			$ 420,000,000
Redemption price (%)			99.50%
Atkore International | Secured Debt | First Lien Term Loan Facility due December 22, 2023 | LIBOR
Debt Instrument [Line Items]
Margin (%)			3.50%
Debt Instrument, Variable Rate Floor			1.00%
Atkore International | Secured Debt | Second Lien Term Loan Facility [Member]
Debt Instrument [Line Items]
Credit agreement			$ 250,000,000
Redemption price (%)			99.00%
Atkore International | Secured Debt | Second Lien Term Loan Facility [Member] | LIBOR
Debt Instrument [Line Items]
Margin (%)			6.75%
Debt Instrument, Variable Rate Floor			1.00%
Atkore International | Secured Debt | New First Lien Term Loan Facility
Debt Instrument [Line Items]
Credit agreement		$ 500,000,000
Redemption price (%)		99.75%
Atkore International | Secured Debt | New First Lien Term Loan Facility | LIBOR
Debt Instrument [Line Items]
Margin (%)		3.00%
Amortization of debt discount (Premium)		$ 0.0100
Atkore International | Secured Debt | New First Lien Term Loan Facility | Alternate Base Rate
Debt Instrument [Line Items]
Margin (%)		2.00%

DEBT - ABL Credit Facility - Na

DEBT - ABL Credit Facility - Narrative (Details) - USD ($)	Aug. 28, 2020	Dec. 22, 2016	Dec. 21, 2016	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2020
Debt Instrument [Line Items]
Gain (loss) on extinguishment of debt				$ (273,000)	$ 0	$ 0
Line of Credit | ABL Credit Facility
Debt Instrument [Line Items]
Debt covenant, percentage to determine specified availability				10.00%			10.00%
Debt covenant, period to determine specified availability				30 days
Atkore International | Domestic Line of Credit | ABL Credit Facility | LIBOR | Minimum
Debt Instrument [Line Items]
Applicable margin (%)	1.75%	1.25%
Atkore International | Domestic Line of Credit | ABL Credit Facility | LIBOR | Maximum
Debt Instrument [Line Items]
Applicable margin (%)	2.25%	1.75%
Atkore International | Domestic Line of Credit | ABL Credit Facility | Alternate Base Rate | Minimum
Debt Instrument [Line Items]
Applicable margin (%)	0.75%	0.25%
Atkore International | Domestic Line of Credit | ABL Credit Facility | Alternate Base Rate | Maximum
Debt Instrument [Line Items]
Applicable margin (%)	1.25%	0.75%
Atkore International | Foreign Line of Credit | ABL Credit Facility
Debt Instrument [Line Items]
Line of credit facility, unused capacity, commitment fee percentage (%)	0.375%		0.25%				0.375%
Atkore International | Foreign Line of Credit | ABL Credit Facility | Bankers Acceptance Rate | Minimum
Debt Instrument [Line Items]
Applicable margin (%)	1.75%	1.25%
Atkore International | Foreign Line of Credit | ABL Credit Facility | Bankers Acceptance Rate | Maximum
Debt Instrument [Line Items]
Applicable margin (%)	2.25%	1.75%
Atkore International | Foreign Line of Credit | ABL Credit Facility | Prime Rate | Minimum
Debt Instrument [Line Items]
Applicable margin (%)	0.75%	0.25%
Atkore International | Foreign Line of Credit | ABL Credit Facility | Prime Rate | Maximum
Debt Instrument [Line Items]
Applicable margin (%)	1.25%	0.75%
Atkore International | Line of Credit | ABL Credit Facility
Debt Instrument [Line Items]
Aggregate commitments				$ 325,000,000			$ 325,000,000
Credit availability				$ 265,899,000	301,882,000		$ 265,899,000
Borrowing base percentage of eligible accounts receivable (%)				85.00%			85.00%
Borrowing base percentage of eligible inventory (plus) (%)				80.00%			80.00%
Debt instrument, covenant, borrowing base, percentage of inventory subject to certain limitations (%)				85.00%			85.00%
Borrowings outstanding				$ 0	$ 0		$ 0
Debt covenant, minimum fixed charge coverage ratio				1			1
Debt covenant, benchmark amount to determine specified availability				$ 22,000,000			$ 22,000,000

FAIR VALUE MEASUREMENTS - Narra

FAIR VALUE MEASUREMENTS - Narrative (Details) - GBP (£) £ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Forward currency contracts	£ 43.3	£ 45	£ 49.1
Measurement Input Expected Term | Foreign Exchange Contract | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative, term of contract	6 months
Measurement Input Expected Term | Foreign Exchange Contract | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative, term of contract	2 years

FAIR VALUE MEASUREMENTS - Asset

FAIR VALUE MEASUREMENTS - Assets and Liabilities Measured at Fair Value (Details) - Recurring $ in Thousands, £ in Millions	Sep. 30, 2020USD ($)	Sep. 30, 2020GBP (£)	Sep. 30, 2019USD ($)	Sep. 30, 2019GBP (£)	Sep. 30, 2018GBP (£)
Assets
Forward currency contracts | £		£ 43.3		£ 45	£ 49.1
Level 1
Assets
Cash equivalents	$ 209,421		$ 72,132
Forward currency contracts	0		0
Liabilities
Forward currency contracts	0		0
Level 2
Assets
Cash equivalents	0		0
Forward currency contracts	2,209		3,420
Liabilities
Forward currency contracts	102		0
Level 3
Assets
Cash equivalents	0		0
Forward currency contracts	0		0
Liabilities
Forward currency contracts	$ 0		$ 0

FAIR VALUE MEASUREMENTS - Estim

FAIR VALUE MEASUREMENTS - Estimated Fair Value of Financial Instruments Not Carried at Fair Value (Details) - Secured Debt - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value	$ 812,120	$ 852,120
Fair value	808,060	853,543
First Lien Term Loan Facility due December 22, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value	812,120	852,120
Fair value	$ 808,060	$ 853,543

COMMITMENTS AND CONTINGENCIES (

COMMITMENTS AND CONTINGENCIES (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2018	Sep. 30, 2019	Mar. 31, 2017
Loss Contingencies [Line Items]
Purchase obligation for fiscal 2021	$ 130,932
Purchase obligation for fiscal 2022	1,828
Purchase obligation thereafter	13
Total settlement for prior cap agreement deductible			$ 13,000
Flood
Loss Contingencies [Line Items]
Estimated loss from unusual or infrequent item	6,046
Insurance recovery amount	5,046
Insurance deductible expense	1,000
Special Products Claims and Other Product Liabilities
Loss Contingencies [Line Items]
Product liability reserves	597		2,424
Special Products Claims and Other Product Liabilities | Minimum
Loss Contingencies [Line Items]
Probable losses	1,000
Special Products Claims and Other Product Liabilities | Maximum
Loss Contingencies [Line Items]
Probable losses	$ 8,000
Special Products Claims
Loss Contingencies [Line Items]
Total settlement for cap agreement deductible			$ 12,000
Antidumping Duty, Malleable Iron Pipe Fittings from China
Loss Contingencies [Line Items]
Loss contingency accrual				$ 7,501
Loss contingency accrual reversal		$ 7,171

GUARANTEES - Narrative (Details

GUARANTEES - Narrative (Details) $ in Thousands	Sep. 30, 2020USD ($)
Workers' compensation and general liability insurance policies
Guarantor Obligations [Line Items]
Guarantees	$ 9,501
Surety bond
Guarantor Obligations [Line Items]
Guarantees	$ 19,468

RELATED PARTY TRANSACTIONS (Det

RELATED PARTY TRANSACTIONS (Details) - USD ($) $ / shares in Units, $ in Thousands, shares in Millions	Jan. 22, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Related Party Transaction [Line Items]
Common stock, par value (in dollars per share)		$ 0.01	$ 0.01
Repurchase of common stock		$ 15,011	$ 24,419	$ 411,775
Related party net sales				$ 6,000
CD&R Allied Holdings, L.P.
Related Party Transaction [Line Items]
Common stock, par value (in dollars per share)	$ 0.01
Treasury stock acquired, average cost per share (in dollars per share)	$ 21.77
Repurchase of common stock	$ 375,000
Common Stock | CD&R Allied Holdings, L.P.
Related Party Transaction [Line Items]
Repurchase of stock (in shares)	17.2

SEGMENT INFORMATION - Narrative

SEGMENT INFORMATION - Narrative (Details)	12 Months Ended
	Sep. 30, 2020segmentemployee	Sep. 30, 2018
Segment Reporting, Revenue Reconciling Item [Line Items]
Number of operating segments	2
Number of reportable segments	2
Employees represented by a union under a collective bargaining agreement (approximately) (%)	24.00%
Number of employees represented by a union under a collective bargaining agreement | employee	350
Sonepar Management US, Inc.
Segment Reporting, Revenue Reconciling Item [Line Items]
Customer percentage of sales or accounts receivable (%)	11.00%	11.00%

SEGMENT INFORMATION - Shared As

SEGMENT INFORMATION - Shared Assets (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 26, 2020	Mar. 27, 2020	Dec. 27, 2019	Sep. 30, 2019	Jun. 28, 2019	Mar. 29, 2019	Dec. 28, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Net sales	$ 477,420	$ 384,899	$ 455,654	$ 447,448	$ 501,710	$ 493,491	$ 469,309	$ 452,028	$ 1,765,421	$ 1,916,538	$ 1,835,139
Capital Expenditures									33,770	34,860	38,501
Total Assets	1,558,525				1,436,995				1,558,525	1,436,995	1,324,060
Electrical Raceway
Segment Reporting Information [Line Items]
Net sales									1,328,721	1,442,278	1,365,067
MP&S
Segment Reporting Information [Line Items]
Net sales									436,700	474,260	470,072
Intersegment Sales
Segment Reporting Information [Line Items]
Net sales									(2,554)	(1,215)	(1,625)
Intersegment Sales | Electrical Raceway
Segment Reporting Information [Line Items]
Net sales									(2,554)	(1,215)	(1,544)
Intersegment Sales | MP&S
Segment Reporting Information [Line Items]
Net sales									0	0	(81)
Operating Segments
Segment Reporting Information [Line Items]
Net sales									1,765,421	1,916,538	1,835,139
Adjusted EBITDA									360,637	362,625	306,599
Operating Segments | Electrical Raceway
Segment Reporting Information [Line Items]
Net sales									1,328,721	1,442,278	1,365,067
Adjusted EBITDA									299,485	292,585	255,260
Capital Expenditures									17,667	19,856	16,389
Total Assets	835,629				871,771				835,629	871,771	751,024
Operating Segments | MP&S
Segment Reporting Information [Line Items]
Net sales									436,700	474,260	470,072
Adjusted EBITDA									61,152	70,040	51,339
Capital Expenditures									13,870	13,934	14,267
Total Assets	215,350				252,708				215,350	252,708	291,164
Unallocated
Segment Reporting Information [Line Items]
Capital Expenditures									2,233	1,070	7,845
Total Assets	$ 507,546				$ 312,516				$ 507,546	$ 312,516	$ 281,872

SEGMENT INFORMATION - Reconcili

SEGMENT INFORMATION - Reconciliation of Operating Segment Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Unallocated expenses	$ (34,002)	$ (38,217)	$ (35,050)
Depreciation and amortization	(74,470)	(72,347)	(66,890)
Interest expense, net	(40,062)	(50,473)	(40,694)
Gain (loss) on extinguishment of debt	(273)	0	0
Restructuring charges	(3,284)	(3,804)	(1,849)
Stock-based compensation	(13,064)	(11,798)	(14,664)
Certain legal matters	0	0	4,833
Transaction costs	(196)	(1,200)	(9,314)
Gain on purchase of business	0	7,384	0
Gain on sale of a business	0	0	27,575
Other	6,712	(7,501)	(4,194)
Income before income taxes	201,998	184,669	166,352
Operating Segments
Segment Reporting Information [Line Items]
Operating segment Adjusted EBITDA	360,637	362,625	306,599
Operating Segments | Electrical Raceway
Segment Reporting Information [Line Items]
Operating segment Adjusted EBITDA	299,485	292,585	255,260
Operating Segments | MP&S
Segment Reporting Information [Line Items]
Operating segment Adjusted EBITDA	$ 61,152	$ 70,040	$ 51,339

SEGMENT INFORMATION - Schedule

SEGMENT INFORMATION - Schedule of Long-lived Assets and Net Sales By Geography (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 26, 2020	Mar. 27, 2020	Dec. 27, 2019	Sep. 30, 2019	Jun. 28, 2019	Mar. 29, 2019	Dec. 28, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	$ 246,882				$ 264,966				$ 246,882	$ 264,966	$ 214,715
Net sales	477,420	$ 384,899	$ 455,654	$ 447,448	501,710	$ 493,491	$ 469,309	$ 452,028	1,765,421	1,916,538	1,835,139
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	203,694				219,614				203,694	219,614	201,101
Net sales									1,563,258	1,689,194	1,651,636
Other Americas
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	146				147				146	147	138
Net sales									26,421	33,485	43,013
Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	41,283				43,207				41,283	43,207	11,090
Net sales									132,299	142,279	90,915
Asia-Pacific
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	$ 1,759				$ 1,998				1,759	1,998	2,386
Net sales									$ 43,443	$ 51,580	$ 49,575

SEGMENT INFORMATION - Schedul_2

SEGMENT INFORMATION - Schedule of Net Sales From External Customers by Product Category (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 26, 2020	Mar. 27, 2020	Dec. 27, 2019	Sep. 30, 2019	Jun. 28, 2019	Mar. 29, 2019	Dec. 28, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Revenue from External Customer [Line Items]
Net sales	$ 477,420	$ 384,899	$ 455,654	$ 447,448	$ 501,710	$ 493,491	$ 469,309	$ 452,028	$ 1,765,421	$ 1,916,538	$ 1,835,139
Electrical Raceway
Revenue from External Customer [Line Items]
Net sales									1,328,721	1,442,278	1,365,067
Electrical Raceway | Metal Electrical Conduit and Fittings
Revenue from External Customer [Line Items]
Net sales									484,476	546,533	517,935
Electrical Raceway | Armored Cable and Fittings
Revenue from External Customer [Line Items]
Net sales									306,460	360,494	336,388
Electrical Raceway | PVC Electrical Conduit & Fittings
Revenue from External Customer [Line Items]
Net sales									308,561	292,243	311,811
Electrical Raceway | Cable Tray & Cable Ladders
Revenue from External Customer [Line Items]
Net sales									193,306	199,303	145,795
Electrical Raceway | Other raceway products
Revenue from External Customer [Line Items]
Net sales									35,918	43,705	53,138
Mechanical Products & Solutions
Revenue from External Customer [Line Items]
Net sales									436,700	474,260	470,072
Mechanical Products & Solutions | Mechanical Pipe
Revenue from External Customer [Line Items]
Net sales									244,902	259,613	253,381
Mechanical Products & Solutions | Other MP&S products
Revenue from External Customer [Line Items]
Net sales									$ 191,798	$ 214,647	$ 216,691

QUARTERLY OPERATING RESULTS (_3

QUARTERLY OPERATING RESULTS (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	Aug. 21, 2019	Sep. 30, 2020	Jun. 26, 2020	Mar. 27, 2020	Dec. 27, 2019	Sep. 30, 2019	Jun. 28, 2019	Mar. 29, 2019	Dec. 28, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Debt Instrument [Line Items]
Net sales		$ 477,420	$ 384,899	$ 455,654	$ 447,448	$ 501,710	$ 493,491	$ 469,309	$ 452,028	$ 1,765,421	$ 1,916,538	$ 1,835,139
Gross profit		147,054	95,813	131,603	116,844	143,722	126,134	117,088	110,256	491,314	497,200	438,084
Net income		$ 54,241	$ 24,078	$ 39,193	$ 34,790	$ 45,997	$ 36,550	$ 29,555	$ 26,949	$ 152,302	$ 139,051	$ 136,645
Net income per share
Basic (in dollars per share)		$ 1.12	$ 0.50	$ 0.81	$ 0.72	$ 0.96	$ 0.77	$ 0.62	$ 0.56	$ 3.15	$ 2.91	$ 2.59
Diluted (in dollars per share)		$ 1.11	$ 0.49	$ 0.80	$ 0.71	$ 0.94	$ 0.75	$ 0.61	$ 0.54	$ 3.10	$ 2.83	$ 2.48
Bargain purchase gain										$ 0	$ 7,384	$ 0
Cor-Tek
Net income per share
Bargain purchase gain	$ 7,384	$ 7,384

SUBSEQUENT EVENTS (Details)

SUBSEQUENT EVENTS (Details) - USD ($) $ in Millions	Oct. 22, 2020	Nov. 19, 2020	Nov. 13, 2020
Subsequent Event [Line Items]
Shares outstanding (in shares)			47,409,886
Subsequent Event
Subsequent Event [Line Items]
Assets purchase price	$ 7.3
Repurchase of stock (in shares)		637,200
Stock repurchased during period		$ 15

SCHEDULE I - CONDENSED FINANC_2

SCHEDULE I - CONDENSED FINANCIAL INFORMATION - Balance Sheets (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Assets
Total Assets	$ 1,558,525	$ 1,436,995	$ 1,324,060
Liabilities and Equity
Total Liabilities	1,180,115	1,204,059
Equity:
Common stock, $0.01 par value, 1,000,000,000 shares authorized, 47,407,023 and 46,955,163 shares issued and outstanding, respectively	475	471
Treasury stock, held at cost, 260,900 and 260,900 shares, respectively	(2,580)	(2,580)
Additional paid-in capital	487,223	477,139
Accumulated deficit	(64,154)	(200,396)
Accumulated other comprehensive loss	(42,554)	(41,698)
Total Equity	378,410	232,936	$ 122,059	$ 360,871
Total Liabilities and Equity	1,558,525	1,436,995
Atkore International Group Inc.
Assets
Investment in subsidiary	378,410	232,936
Total Assets	378,410	232,936
Liabilities and Equity
Total Liabilities	0	0
Equity:
Additional paid-in capital	487,223	477,139
Total Equity	378,410	232,936
Total Liabilities and Equity	$ 378,410	$ 232,936

SCHEDULE I - CONDENSED FINANC_3

SCHEDULE I - CONDENSED FINANCIAL INFORMATION - Balance Sheets (Parenthetical) (Details) - $ / shares	Sep. 30, 2020	Sep. 30, 2019
Condensed Balance Sheet Statements, Captions [Line Items]
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (shares)	1,000,000,000	1,000,000,000
Common stock, shares issued (shares)	47,407,023	46,955,163
Common stock, shares outstanding (shares)	47,407,023	46,955,163
Treasury stock (shares)	260,900	260,900
Atkore International Group Inc.
Condensed Balance Sheet Statements, Captions [Line Items]
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (shares)	1,000,000,000	1,000,000,000
Common stock, shares issued (shares)	47,407,023	46,955,163
Common stock, shares outstanding (shares)	47,407,023	46,955,163
Treasury stock (shares)	260,900	260,900

SCHEDULE I - CONDENSED FINANC_4

SCHEDULE I - CONDENSED FINANCIAL INFORMATION - Condensed Statements of Operations (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 26, 2020	Mar. 27, 2020	Dec. 27, 2019	Sep. 30, 2019	Jun. 28, 2019	Mar. 29, 2019	Dec. 28, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Condensed Income Statements, Captions [Line Items]
Net income	$ 54,241	$ 24,078	$ 39,193	$ 34,790	$ 45,997	$ 36,550	$ 29,555	$ 26,949	$ 152,302	$ 139,051	$ 136,645
Other comprehensive (loss) income of subsidiary, net of tax									(856)	(22,927)	1,544
Comprehensive income									151,446	116,124	138,189
Atkore International Group Inc.
Condensed Income Statements, Captions [Line Items]
Net income									152,302	139,051	136,645
Other comprehensive (loss) income of subsidiary, net of tax									(856)
Comprehensive income									$ 151,446	$ 116,124	$ 138,189

SCHEDULE I - CONDENSED FINANC_5

SCHEDULE I - CONDENSED FINANCIAL INFORMATION - Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Cash Flows from Operating Activities:
Net cash provided by operating activities	$ 248,762	$ 209,694	$ 145,703
Cash Flows from Investing Activities:
Net cash (used for) provided by investing activities	(27,513)	(133,101)	2,514
Cash Flows from Financing Activities:
Issuance of common stock, net of taxes withheld	(2,972)	7,374	20,110
Repurchase of common stock	(15,011)	(24,419)	(411,775)
Net cash used for financing activities	(61,179)	(78,180)	(65,931)
Increase (decrease) in cash and cash equivalents	161,056	(3,247)	80,944
Atkore International Group Inc.
Cash Flows from Operating Activities:
Net cash provided by operating activities	0	0	0
Cash Flows from Investing Activities:
Distribution received from subsidiary	17,983	24,419	411,775
Distribution paid to subsidiary	0	(7,374)	(20,110)
Net cash (used for) provided by investing activities	17,983	17,045	391,665
Cash Flows from Financing Activities:
Issuance of common stock, net of taxes withheld		7,374	20,110
Repurchase of common stock	(15,011)	(24,419)	(411,775)
Net cash used for financing activities	(17,983)	(17,045)	(391,665)
Increase (decrease) in cash and cash equivalents	0	0	0
Cash and cash equivalents:
Cash and cash equivalents at beginning of period	0	0	0
Cash and cash equivalents at end of period	$ 0	$ 0	$ 0

SCHEDULE I - CONDENSED FINANC_6

SCHEDULE I - CONDENSED FINANCIAL INFORMATION - Narrative (Details) - USD ($) $ in Thousands, shares in Millions	Apr. 09, 2014	Mar. 06, 2014	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Dec. 22, 2010
Condensed Financial Statements, Captions [Line Items]
Payment to redeem shares	$ 250,000
Atkore International Group Inc.
Condensed Financial Statements, Captions [Line Items]
Distribution received from subsidiary			$ 17,983	$ 24,419	$ 411,775
Common Stock
Condensed Financial Statements, Captions [Line Items]
Non-binding letter of intent to acquire common stock (shares)		40.3
Expense related to share redemption	$ 2,000
Atkore International | CD&R
Condensed Financial Statements, Captions [Line Items]
Preferred stock, percentage of outstanding capital stock (on an as-converted basis) (%)						51.00%
Common stock, percentage of outstanding capital stock (%)						49.00%

SCHEDULE II - VALUATION AND Q_2

SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Accounts Receivable Allowance for Doubtful Accounts
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	$ (2,608)	$ (1,762)	$ (1,239)
Additional (Charges)/Benefit to Income	(1,990)	(970)	(389)
Write offs and Other	1,431	124	(134)
Balance at End of Year	(3,167)	(2,608)	(1,762)
Deferred Tax Valuation Allowance
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	(7,764)	(7,427)	(9,512)
Additional (Charges)/Benefit to Income	(2,047)	(346)	428
Write offs and Other	(392)	9	1,657
Balance at End of Year	$ (10,203)	$ (7,764)	$ (7,427)