Atkore (ATKR) 10-K 22 Nov 19 2019 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2019	Nov. 18, 2019	Mar. 29, 2019
Cover page.
Document Type	10-K
Document Annual Report	true
Document Transition Report	false
Entity File Number	001-37793
Entity Registrant Name	Atkore International Group Inc.
Entity Central Index Key	0001666138
Document Period End Date	Sep. 30, 2019
Document Fiscal Year Focus	2019
Amendment Flag	false
Document Fiscal Period Focus	FY
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Shell Company	false
Document Fiscal Year Focus	--09-30
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 Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		46,983,702
Entity Public Float			$ 983.1
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 2020 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, 2019 .

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Net sales	$ 1,916,538	$ 1,835,139	$ 1,503,934
Cost of sales	1,419,338	1,397,055	1,142,664
Gross profit	497,200	438,084	361,270
Selling, general and administrative	240,660	226,282	182,910
Intangible asset amortization	32,876	32,104	22,407
Operating income	223,664	179,698	155,953
Interest expense, net	50,473	40,694	26,598
Loss on extinguishment of debt	0	0	9,805
Other income, net	(11,478)	(27,348)	(6,575)
Income before income taxes	184,669	166,352	126,125
Income tax expense	45,618	29,707	41,486
Net income	$ 139,051	$ 136,645	$ 84,639
Net income per share
Basic (in dollars per share)	$ 2.91	$ 2.59	$ 1.33
Diluted (in dollars per share)	$ 2.83	$ 2.48	$ 1.27

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 139,051	$ 136,645	$ 84,639
Change in foreign currency translation adjustment	(7,490)	(2,853)	1,221
Change in unrecognized (loss) income related to pension benefit plans, net of tax benefit (expense) of $5,253, ($1,362) and ($3,356), respectively	(15,437)	4,397	6,747
Total other comprehensive (loss) income	(22,927)	1,544	7,968
Comprehensive income	$ 116,124	$ 138,189	$ 92,607

Consolidated Statements of Co_2

Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Change in unrecognized loss related to pension benefit plans, tax benefit (expense)	$ 5,253	$ (1,362)	$ (3,356)

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018
Current Assets:
Cash and cash equivalents	$ 123,415	$ 126,662
Accounts receivable, less allowance for doubtful accounts of $2,608 and $1,762, respectively	315,353	265,147
Inventories, net	226,090	221,753
Prepaid expenses and other current assets	34,679	33,576
Total current assets	699,537	647,138
Property, plant and equipment, net	260,703	213,108
Intangible assets, net	285,684	291,916
Goodwill	186,231	170,129
Deferred income taxes	577	162
Non-trade receivables	4,263	1,607
Total Assets	1,436,995	1,324,060
Current Liabilities:
Short-term debt and current maturities of long-term debt	0	26,561
Accounts payable	150,681	156,525
Income tax payable	2,157	542
Accrued compensation and employee benefits	35,770	33,350
Customer liabilities	44,983	3,377
Other current liabilities	53,943	52,392
Total current liabilities	287,534	272,747
Long-term debt	845,317	877,686
Deferred income taxes	19,986	16,510
Other long-term tax liabilities	3,669	1,443
Pension liabilities	34,509	17,075
Other long-term liabilities	13,044	16,540
Total Liabilities	1,204,059	1,202,001
Equity:
Common stock, $0.01 par value, 1,000,000,000 shares authorized, 46,955,163 and 47,079,645 shares issued and outstanding, respectively	471	472
Treasury stock, held at cost, 260,900 and 260,900 shares, respectively	(2,580)	(2,580)
Additional paid-in capital	477,139	457,978
Accumulated deficit	(200,396)	(317,373)
Accumulated other comprehensive loss	(41,698)	(16,438)
Total Equity	232,936	122,059
Total Liabilities and Equity	$ 1,436,995	$ 1,324,060

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018
Statement of Financial Position [Abstract]
Accounts receivable, less allowance for doubtful accounts of $2,608 and $1,762, respectively	$ 2,608	$ 1,762
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)	46,955,163	47,079,645
Common stock, shares outstanding (shares)	46,955,163	47,079,645
Treasury stock (shares)	260,900	260,900

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Operating activities
Net income	$ 139,051	$ 136,645	$ 84,639
Adjustments to reconcile net income to net cash provided by operating activities
Gain on sale of a business	0	(27,575)	(5,093)
Depreciation and amortization	72,347	66,890	54,727
Amortization of debt issuance costs and original issue discount	1,804	1,542	1,446
Deferred income taxes	(796)	(9,008)	938
Loss on extinguishment of debt	0	0	9,805
Provision for losses on accounts receivable and inventory	4,656	7,241	1,333
Stock-based compensation expense	11,798	14,664	12,788
Gain on purchase of business	(7,384)	0	0
Other adjustments to net income	(1,938)	2,023	896
Changes in operating assets and liabilities, net of effects from acquisitions
Accounts receivable	6,026	(44,419)	(13,445)
Inventories	9,002	(36,528)	(10,301)
Prepaid expenses and other current assets	(3,054)	(1,762)	(3,074)
Accounts payable	(21,981)	31,667	8,673
Income taxes	4,511	(3,179)	(9,138)
Accrued and other liabilities	(2,782)	7,243	(11,232)
Other, net	(1,566)	259	(1,308)
Net cash provided by operating activities	209,694	145,703	121,654
Investing activities
Capital expenditures	(34,860)	(38,501)	(25,122)
Proceeds from sale of properties, plant and equipment	80	349	100
Proceeds from divestiture of business	0	42,631	0
Proceeds from sale of assets held for sale	0	0	3,024
Acquisitions of businesses, net of cash acquired	(97,999)	(3,467)	(183,923)
Other, net	(322)	1,502	88
Net cash (used for) provided by investing activities	(133,101)	2,514	(205,833)
Financing activities
Borrowings under credit facility	39,000	358,000	97,000
Repayments under credit facility	(39,000)	(443,000)	(12,000)
Repayments of short-term debt	(20,980)	(8,150)	(4,200)
Issuance of long-term debt	0	426,217	498,750
Repayments of long-term debt	(40,000)	(1,217)	(641,100)
Issuance of common stock	7,374	20,110	12,168
Repurchase of common stock	(24,419)	(411,775)	(13,938)
Payments for debt financing costs and fees	0	(5,955)	(4,375)
Other, net	(155)	(161)	(65)
Net cash used for financing activities	(78,180)	(65,931)	(67,760)
Effects of foreign exchange rate changes on cash and cash equivalents	(1,660)	(1,342)	(2,622)
Increase (decrease) in cash and cash equivalents	(3,247)	80,944	(154,561)
Cash and cash equivalents at beginning of period	126,662	45,718	200,279
Cash and cash equivalents at end of period	123,415	126,662	45,718
Supplementary Cash Flow information
Interest paid	49,879	39,898	26,131
Income taxes paid, net of refunds	38,698	41,601	49,813
Capital expenditures, not yet paid	$ 3,719	$ 916	$ 1,330

Consolidated Statements of Shar

Consolidated Statements of Shareholders' Equity - USD ($) shares in Thousands, $ in Thousands		Total	Common Stock	Treasury Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)
Beginning balance (shares) at Sep. 30, 2016			62,458
Beginning balance at Sep. 30, 2016		$ 257,246	$ 626	$ (2,580)	$ 398,292	$ (113,142)	$ (25,950)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		84,639				84,639
Other comprehensive income (loss)		7,968					7,968
Stock-based compensation		12,788			12,788
Issuance of common stock (shares)			1,628
Issuance of common stock		12,168	$ 16		12,152
Repurchase of common stock (shares)			(781)
Repurchase of common stock		(13,938)	$ (8)			(13,930)
Ending balance (shares) at Sep. 30, 2017			63,305
Ending 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	$ 471	$ (2,580)	$ 477,139	$ (200,396)	$ (41,698)
[1]	(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, 2019
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"). The Transactions — 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). The preferred stock is entitled to a 12% fixed, cumulative dividend paid quarterly ("Preferred Dividends") and dividends on an as-converted basis when declared on common stock ("Participating Dividends"). 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. The aforementioned transactions described in this paragraph are referred to herein as the "Transactions." 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 of September 26, 2014, Common Stock is the Company’s sole issued and outstanding class of securities. Initial Public Offering — On June 9, 2016 , the Company's Registration Statement on Form S-1 relating to an initial public offering ("IPO") of its common stock was declared effective by the United States Securities and Exchange Commission ("SEC") and on June 15, 2016 , we completed the IPO at a price to the public of $16.00 per share. In connection with the IPO, the CD&R Investor, an affiliate of CD&R, sold an aggregate of 12,000,000 shares of its common stock. In a series of secondary offerings during fiscal 2017, the CD&R Investor sold an aggregate 19,550,000 shares of the Company's common stock. The CD&R Investor received all of the net proceeds and bore all commissions and discounts from the sale of our common stock. The Company did not receive any proceeds from the IPO or any of the secondary offerings. 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. Share repurchases will be conducted under the program in the open market and through broker negotiated purchases in compliance with Rule 10b-18 of the Securities Exchange Act of 1934, as amended, and subject to market conditions, applicable legal requirements, and other relevant factors. This share repurchase program does not obligate the Company to acquire any particular amount of common stock, and it may be terminated at any time at the Company's discretion. In January 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 million, 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 completed its exit from its investment in the Company, and, as of September 30, 2018, no longer owned any of the Company's common 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, 2019, there were $50.0 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 end on the last Friday in December, March and June. 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 production equipment 1 to 20 years Support and testing machinery and equipment 2 to 10 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 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. In the fourth quarter of 2017, the Company adopted Accounting Standards Update ("ASU") 2017-04, "Intangibles - Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment." This guidance eliminates the requirement to determine the fair value of individual assets and liabilities of a reporting unit to measure goodwill impairment. Under the amendments in the new guidance, the quantitative goodwill impairment testing is performed by comparing the fair value of the reporting unit with its carrying amount and recognizing an impairment charge for the amount by which the carrying amount exceeds the reporting unit's fair value. For fiscal 2019 and fiscal 2018, the Company performed a quantitative impairment assessment for goodwill. 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) a market approach using a transaction analysis. 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. Fair value under the guideline public company method is determined for each unit by applying market multiples for comparable public companies to the unit’s financial results. 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 unit by applying market multiples for comparable public companies to the unit’s financial results. Fair value under the transactional analysis 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 transactional analysis calculations are the assumptions used in determining the reporting unit's comparable public companies, comparable transactions and the selection of the market multiples. For fiscal 2017, the Company determined through qualitative assessment ("Step 0") that it was not “more likely than not” that the fair values of its reporting units were less than their carrying values; as a result, the Company was not required to perform the quantitative impairment test. The Company did not record any goodwill impairments in fiscal 2019, 2018 or 2017. 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 — 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 — 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. 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 2014-09 Revenue from Contracts with Customers and subsequent amendments The Accounting Standards Update ("ASU") provides guidance for revenue recognition. The update's core principle is that a company will recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. In doing so, companies will need to use more judgment and make more estimates than under current guidance. Examples of the use of judgments and estimates may include identifying performance obligations in the contract, estimating the amount of variable consideration to include in the transaction price and allocating the transaction price to each separate performance obligation. The update also requires more detailed disclosures to enable users of financial statements to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. The update provides for two transition methods to the new guidance: a full retrospective approach and a modified retrospective approach. The Company adopted the guidance in the first quarter of 2019 using the modified retrospective method. See Note 2, "Revenue from Contracts with Customers" for further detail. 2 2019 2018-02 Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income The ASU provided entities with the option to reclassify tax effects stranded in accumulated other comprehensive income as a result of the "H.R.1", also known as the "Tax Cuts and Jobs Act" ("TCJA") to retained earnings. The Company elected to adopt the guidance early in the quarter ended March 29, 2019. As a result of adoption of the ASU, the Company reclassified $2,333 of stranded tax benefits related to its pension plans out of Accumulated other comprehensive loss and into Accumulated deficit for the quarter ended March 29, 2019. The Company's policy is to release the tax effects as the related amounts in other comprehensive income are recognized in net income. 11 2019 2018-07 Improvements to Nonemployee Share-Based Payment Accounting. The ASU simplifies the accounting for share-based payments to nonemployees by aligning it with the accounting for share-based payments to employees, with certain exceptions. The Company elected to adopt the guidance early in the quarter ended June 28, 2019. There was no material impact to the consolidated financial statements as a result of the adoption of ASU 2018-07. 2019 2018-13 Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement The ASU amends ASC 820 to add, remove and clarify disclosure requirements related to fair value measurements. The Company elected to adopt the guidance early in the quarter ended June 28, 2019. There was no material impact to the consolidated financial statements as a result of the adoption of ASU 2018-13. 2019 A summary of guidance not yet adopted are as follows: Guidance not yet adopted ASU Description of ASU Impact to Atkore Effective 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 will adopt the new lease guidance in the first quarter of fiscal 2020 utilizing the modified retrospective transition method and will not restate comparative periods. We plan to elect the package of practical expedients permitted under the transition guidance within the ASU, which allows us to carry forward prior conclusions about lease identification, classification and initial direct costs for leases entered into prior to adoption. The new standard also provides practical expedients for an entity’s ongoing accounting. For leases with a term of 12 months or less, we plan to elect the short-term lease exemption, which allows us to not recognize right-of-use assets or lease liabilities for qualifying leases existing at transition and new leases we may enter into in the future. To assist with the ongoing accounting under ASC 842, we have implemented a new lease accounting system. The Company is continuing to assess the impact of the ASU on our consolidated financial statements, required disclosures, and changes to internal controls. Based on the work completed, the Company expects to recognize lease liabilities ranging from $42 million to $52 million, with corresponding right-of-use assets of the same amount. 2020 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. Under evaluation. 2021 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. Under evaluation. 2021

Revenue from Contracts with Cus

Revenue from Contracts with Customers	12 Months Ended
	Sep. 30, 2019
Revenue from Contract with Customer [Abstract]
Revenue from Contracts with Customers	2. REVENUE FROM CONTRACTS WITH CUSTOMERS In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers, and issued subsequent amendments to the initial guidance in August 2015, March 2016, April 2016, May 2016, and December 2016 within ASU 2015-14, ASU 2016-08, ASU 2016-10 and ASU 2016-20, respectively. The core principle of this new revenue recognition guidance is that a company will recognize revenue when promised goods or services are transferred to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. The new guidance defines a five-step process to implement this core principle. The new guidance also requires more detailed disclosures to enable users of financial statements to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. The new guidance provides for two transition methods, a full retrospective approach and a modified retrospective approach. The Company adopted the new guidance on October 1, 2018 utilizing the modified retrospective method of adoption for contracts not completed at the adoption date and determined there were no changes required to its historical reported revenues and earnings as a result of the adoption. The impacts to the consolidated financial statements consist of balance sheet reclassifications including amounts associated with the changes in the classification of reserves related to volume rebates and returns reserves. The Company has also enhanced its disclosures of revenue to comply with the new guidance. The impact to the Company’s financial statements as of September 30, 2019 was as follows: As of September 30, 2019 Balance Sheet As Reported Balances before adoption of ASC 606 Effect of Adoption Higher/(Lower) Accounts Receivable, net 315,353 271,900 43,453 Customer liabilities 44,983 1,530 43,453 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.

Acquisitions

Acquisitions	12 Months Ended
	Sep. 30, 2019
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. 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 condensed 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. Fiscal 2017 On September 29, 2017, AII acquired all of the outstanding stock of Calpipe Industries, LLC ("Calpipe"), a market leader for electrical conduit systems for corrosive environments and bollards for high security, access control and architectural environments. The Calpipe acquisition expanded the Company's product portfolio by adding stainless steel and PVC coated conduit systems, introduced security bollards to the Company's offerings and further complemented the Company's existing Electrical Raceway product offerings. On September 1, 2017, Unistrut, Ltd, a wholly-owned indirect subsidiary of the Company, acquired all of the outstanding stock of Flexicon Limited ("Flexicon"), a leading global manufacturer of metallic and non-metallic flexible cable protection systems that carries many international and market product approvals and serves the industrial, commercial and infrastructure sectors in more than 55 countries. On May 18, 2017, Unistrut, Ltd, a wholly-owned indirect subsidiary of the Company acquired all of the outstanding stock of Marco Cable Management ("Marco"), a leading designer and manufacturer of wire basket cable tray, PVC trunking and aluminum power poles. Marco's product portfolio adds value to the Company's electrical distribution partners in the U.K. and expands the Company's presence in the U.K. and the rest of Europe. The acquisitions of Marco and Flexicon were funded using cash-on-hand. The acquisition of Calpipe was partially funded using borrowings from the ABL Credit Facility. The Company incurred approximately $558 and $1,676 for fiscal 2017 acquisition-related expenses, which were recorded as a component of selling, general and administrative expenses for the years ended September 30, 2018 and September 30, 2017 , respectively. 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 As of September 30, 2019, with the exception of Vergokan, the purchase price allocation for the remaining acquisitions has not been finalized as the Company is refining its fair value estimates for working capital, intangible assets and fixed assets. 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 2017: (in thousands) Calpipe Other Total Fair value of consideration transferred: Cash consideration $ 110,155 $ 87,649 $ 197,804 Working capital adjustment 120 — 120 Purchase price payable 2,278 — 2,278 Settlement of pre-existing relationship (382 ) — (382 ) Total consideration transferred 112,171 87,649 199,820 Fair value of assets acquired and liabilities assumed: Cash 5,051 8,830 13,881 Accounts receivable 10,369 7,589 17,958 Inventories 18,360 7,221 25,581 Intangible assets 54,860 40,100 94,960 Fixed assets 3,245 11,242 14,487 Accounts payable (1,601 ) (1,550 ) (3,151 ) Other (8,342 ) (11,558 ) (19,900 ) Net assets acquired 81,942 61,874 143,816 Excess purchase price attributed to goodwill acquired $ 30,229 $ 25,775 $ 56,004 From the date of the Company's preliminary purchase price allocation for the 2017 acquisitions through September 30, 2018, the Company recorded adjustments based on our valuation procedures for our acquisitions that resulted in an increase to goodwill of $24,527 . The adjustments were primarily attributable to a decrease of $18,793 in the value allocated to acquired assets, primarily intangible assets and working capital, partially offset by an increase in the value of property, plant and equipment of $2,536 . Additionally, an adjustment of $8,270 was recorded for deferred tax liabilities as a result of the purchase accounting adjustments. Goodwill recognized from the Calpipe acquisition is tax-deductible and is amortized over 15 years for income tax purposes. Goodwill recognized from the other acquisitions was non-deductible for tax purposes. The goodwill consists largely of the synergies and economies of scale from integrating these companies with existing businesses. See Note 14, ''Goodwill and Intangible Assets'' . The following table summarizes the fair value of intangible assets acquired in fiscal 2017 as of the acquisition dates: Calpipe Other ($ in thousands) Fair Value Weighted Average Useful Life (Years) Fair Value Weighted Average Useful Life (Years) Customer relationships $ 50,680 10.0 $ 37,341 10.0 Other 4,180 8.5 2,759 8.1 Total intangible assets $ 54,860 9.9 $ 40,100 9.9 The following table presents unaudited pro forma results of operations for the fiscal year ended September 30, 2017 as if the Calpipe acquisition had occurred as of the first day of the fiscal 2017 period: Fiscal Year Ended (in thousands) September 30, 2017 Pro forma net sales $ 1,575,801 Pro forma net income 91,362 The pro forma condensed financial information is presented for illustrative purposes only and does not indicate the actual financial results of the Company if the closing of the Calpipe acquisition had been completed on September 26, 2015, nor is it indicative of the results of operations in future periods. Included in the unaudited pro forma financial information for the fiscal year ended September 30, 2017 were pro forma adjustments to reflect the results of operations of Calpipe as well as the impact of amortizing certain acquisition accounting adjustments such as the nonrecurring inventory step up adjustment of $2,159 . The pro forma financial information neither indicates the impact of possible business model changes nor considers any potential impact of current market conditions, expense efficiencies or other factors. Net sales and net income of the other 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, 2019
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, 2019
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, 2019 September 30, 2018 September 30, 2017 Service cost $ — $ — $ 2,049 Interest cost 4,665 4,098 3,793 Expected return on plan assets (6,371 ) (6,415 ) (6,601 ) Amortization of actuarial loss 101 343 1,303 Net periodic (benefit) cost $ (1,605 ) $ (1,974 ) $ 544 The weighted-average assumptions used to determine net periodic pension (benefit) cost during the period were as follow: September 30, 2019 September 30, 2018 September 30, 2017 Discount rate 4.2 % 3.7 % 3.5 % Expected return on plan assets 6.3 % 6.4 % 7.0 % 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, 2017 $ 128,852 Interest cost 4,098 Actuarial gain (7,130 ) Benefits and administrative expenses paid (4,863 ) 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 Change in plan assets: Balance as of September 30, 2017 $ 103,613 Actual return on plan assets 4,701 Employer contributions 1,003 Benefits and administrative expenses paid (4,863 ) 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 Funded status: Funded status as of September 30, 2018 $ (16,503 ) Funded status as of September 30, 2019 $ (34,509 ) (in thousands) September 30, 2019 September 30, 2018 Amounts recognized in the consolidated balance sheets consist of: Pension assets $ — $ 572 Pension liabilities (34,509 ) (17,075 ) Net amount recognized $ (34,509 ) $ (16,503 ) Amounts recognized in accumulated other comprehensive loss (before income taxes) consist of: Net actuarial loss $ (33,035 ) $ (12,344 ) Total loss recognized $ (33,035 ) $ (12,344 ) Weighted-average assumptions used to determine pension benefit obligations at year end: Discount rate 3.1 % 4.2 % Rate of compensation increase N/a N/a The following table summarizes the defined benefit pension plans with accumulated benefit obligations in excess of plan assets: (in thousands) September 30, 2019 September 30, 2018 Accumulated benefit obligation $ 137,959 $ 105,414 Fair value of plan assets 103,451 88,339 The following table summarizes the defined benefit pension plans with projected benefit obligations in excess of plan assets: (in thousands) September 30, 2019 September 30, 2018 Projected benefit obligation $ 137,959 $ 105,414 Fair value of plan assets 103,451 88,339 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, 2019 September 30, 2018 Equity securities 58% 59% Debt securities 40% 39% Cash and cash equivalents 2% 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, 2019 , 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, 2019 September 30, 2018 Level 1 Level 2 Total Level 1 Level 2 Total United States equity securities $ 32,545 $ 6,429 $ 38,974 $ 32,447 $ 6,354 $ 38,801 Non-U.S. equity securities 18,182 2,543 20,725 20,164 2,599 22,763 Fixed income securities 35,214 6,815 42,029 34,111 6,678 40,789 Cash and cash equivalents 1,723 — 1,723 2,101 — 2,101 Total $ 87,664 $ 15,787 $ 103,451 $ 88,823 $ 15,631 $ 104,454 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, 2019 and September 30, 2018 , 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 $1,079 and $1,003 to its pension plans for the fiscal years ended September 30, 2019 and September 30, 2018 . The Company anticipates that it will contribute at least the minimum required contribution of $2,123 to its pension plans in fiscal 2020 . Benefit payments, which reflect future expected service as appropriate, are expected to be paid in each fiscal year as follows: (in thousands) 2020 $ 5,841 2021 6,159 2022 6,442 2023 6,670 2024 6,903 2025 to 2029 37,071 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 $3,572 , $3,600 and $3,000 for the fiscal years ended September 30, 2019 , September 30, 2018 and September 30, 2017 , respectively. Multi-Employer Plan — The Company has a liability of $5,658 as of September 30, 2019 and $5,934 as of September 30, 2018 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, 2019
Share-based Payment Arrangement [Abstract]
Stock Incentive Plan	6. STOCK INCENTIVE PLAN On June 10, 2016, the Company's board of directors adopted the Atkore International Group Inc. 2016 Omnibus Incentive Plan (the " Omnibus Incentive Plan "). The 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 Omnibus Incentive Plan replaces and succeeds the Atkore International Group Inc. Stock Incentive Plan (the "Stock Incentive Plan"). The Company no longer grants awards from the Stock Incentive Plan. Awards previously granted under the Stock Incentive Plan were unaffected by the termination. A maximum of 1.6 million shares of common stock is reserved for issuance under the Omnibus Incentive Plan. 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 $11,798 , $14,664 and $12,788 for fiscal years 2019 , 2018 and 2017 , respectively. The total income tax benefit recognized for share-based compensation arrangements was $2,949 , $3,681 and $4,731 for fiscal years 2019 , 2018 and 2017 , 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 Topic 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 and modified were as follows: Fiscal Year Ended September 30, 2019 September 30, 2018 September 30, 2017 Expected dividend yield — % — % — % Expected volatility 45 % 40 % 40 % Range of risk-free interest rates 2.94% - 3.00% 2.16% - 2.38% 1.95 % Range of expected option lives 6 to 10 years 1 to 6 years 6 years 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 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, 2016 to September 30, 2019 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, 2016 6,664 $ 7.76 Granted 171 21.45 $ 8.84 Exercised (1,629 ) 7.47 $ 25,757 Forfeited (48 ) 9.02 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 5.72 Exercisable as of September 30, 2019 1,301 $ 9.46 $ 27,185 4.63 As of September 30, 2019 , there was $2,575 of total unrecognized compensation expense related to non-vested options granted expected to be recognized over a weighted-average period of approximately 1.50 years. The total fair value of shares vested during fiscal years 2019 , 2018 and 2017 was $3,038 , $9,757 and $10,696 , respectively. Cash received from stock option exercises for the fiscal years 2019 , 2018 and 2017 was $7,374 , $20,110 and $12,168 , respectively. The actual tax benefit for the tax deductions from stock option exercises totaled $4,031 , $11,478 and $9,530 , respectively, for fiscal years 2019 , 2018 and 2017 . The Company does not settle any option exercises, under its current stock incentive plan, in cash. Restricted Stock Units Generally, RSUs granted under the 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, 2019 were as follows: Shares Weighted-average grant-date fair value Nonvested as of September 30, 2016 — $ — Granted 443 19.68 Forfeited (8 ) 21.45 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 As of September 30, 2019 , there was $ 6,460 of total unrecognized compensation expense related to non-vested RSUs granted, expected to be recognized over a weighted-average period of approximately 1.7 years. The total fair value of RSUs vested during fiscal years 2019 , 2018 and 2017 was $ 4,271 , $1,969 and $0 , respectively. Performance Share Units On November 30, 2016, November 28, 2017 and November 27, 2018, 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 2019, 2018 and 2017, the closing stock price on the date of grant was $17.90 , $20.01 and $21.45 , 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 and modified during the fiscal year ended September 30, 2019 were as follows: September 30, 2019 September 30, 2018 September 30, 2017 Expected dividend yield — % — % — % Range of expected volatility 20.69 - 74.26 17.21 - 79.15 17.55 - 75.55 Risk free interest rates 2.84 % 1.83% - 2.54% 1.35 % Expected life 3 years 1 -3 years 3 years Fair value $ 18.84 $ 13.90 - 26.68 $ 29.53 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, 2019 were as follows: Shares Weighted-average grant-date fair value Nonvested as of September 30, 2016 — $ — Granted 184 23.87 Forfeited (3 ) 23.87 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 As of September 30, 2019 , there was $4,020 of total unrecognized compensation expense related to non-vested PSUs granted, expected to be recognized over a weighted-average period of approximately 1.64 years. There were no PSUs vested during fiscal years 2019 , 2018 and 2017 .

Restructuring Charges

Restructuring Charges	12 Months Ended
	Sep. 30, 2019
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 (a) Severance (a) Other (a) Severance Total Balance as of September 30, 2016 $ 841 $ — $ — $ 539 $ — $ 1,380 Charges 527 439 422 63 71 1,522 Utilization (917 ) (209 ) (166 ) (556 ) (71 ) (1,919 ) Reversals — (230 ) — (36 ) — (266 ) Exchange rate effects (2 ) — 22 — — 20 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 (a) 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, 2019 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, 2019 September 30, 2018 September 30, 2017 Total restructuring charges, net $ 3,804 $ 1,849 $ 1,256

Other Income, Net

Other Income, Net	12 Months Ended
	Sep. 30, 2019
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, 2019 September 30, 2018 September 30, 2017 Gain on purchase of business $ (7,384 ) $ — $ — Gain on sale of a business — (27,575 ) — Gain on sale of joint venture — — (5,774 ) Undesignated foreign currency derivative instruments (5,384 ) (121 ) 2,741 Foreign exchange loss (gain) on intercompany loans 2,975 1,500 (2,038 ) Debt modification costs — 892 — Pension-related benefits (1,606 ) (1,975 ) (1,504 ) Other (79 ) (69 ) — Other income, net $ (11,478 ) $ (27,348 ) $ (6,575 )

Income Taxes

Income Taxes	12 Months Ended
	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Income Taxes	9. INCOME TAXES 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. The value of the Company’s net deferred tax liability on the balance sheet decreased as a result of the newly enacted tax rates creating a one-time tax benefit to the Company; the preliminary analysis of the impact, using December 29, 2017 values, was an estimated decrease to the net deferred tax liability of $ 4,758 , which was recognized in the first quarter of fiscal 2018. The SEC Staff Accounting Bulletin No. 118 allowed for a measurement period of up to one year from the date of enactment; during the course of the fiscal year ended September 30, 2018, the Company recorded an adjustment to the re-measurement of deferred tax liabilities of an additional $708 benefit as a result of updated estimates. For the period ended December 28, 2018, the Company finalized the re-measurement with no additional adjustments. The Company has an accumulated earnings and profits deficit in the foreign jurisdictions in which it operates. The Company completed its calculation and did not have an income tax liability from the one-time transition tax on the deemed repatriation of its foreign earnings. The GILTI provision of TCJA requires certain income earned by controlled foreign corporations ("CFCs") to be included currently in the gross income of the CFC's 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, 2019 , September 30, 2018 and September 30, 2017 consisted of the following: (in thousands) September 30, 2019 September 30, 2018 September 30, 2017 Components of income before income taxes: United States $ 170,780 $ 155,453 $ 116,580 Non-U.S 13,889 10,899 9,545 Income before income taxes $ 184,669 $ 166,352 $ 126,125 Income tax expense: Current: United States: Federal $ 31,431 $ 26,586 $ 33,127 State 9,800 8,662 4,246 Non-U.S: 5,183 3,467 3,175 Current income tax expense $ 46,414 $ 38,715 $ 40,548 Deferred: United States: Federal $ 1,123 $ (7,149 ) $ 224 State (340 ) (1,793 ) 469 Non-U.S: (1,579 ) (66 ) 245 Deferred income (benefit) tax expense (796 ) (9,008 ) 938 Income tax expense $ 45,618 $ 29,707 $ 41,486 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, 2019 September 30, 2018 September 30, 2017 Statutory federal tax 21 % 25 % 35 % Adjustments to reconcile to the effective income tax rate: State income taxes 4 % 3 % 3 % Change in tax rates — % (3 )% — % Domestic manufacturing deduction — % (2 )% (2 )% Indemnified uncertain tax benefits — % (2 )% — % Stock-based compensation (1 )% (3 )% (3 )% Other 1 % — % — % Effective income tax rate 25 % 18 % 33 % 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. The Company's effective tax rate for fiscal 2017 differs from the statutory rate primarily due to a $3,589 excess tax benefit associated with the exercise of stock options, a $2,761 tax benefit for domestic manufacturing deduction, partially offset by $3,459 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, 2019 September 30, 2018 Deferred tax assets: Accrued liabilities and reserves $ 32,664 $ 32,099 Tax loss and credit carryforwards 13,296 12,435 Postretirement benefits 10,315 4,317 Inventory 8,813 11,220 Other 916 218 $ 66,004 $ 60,289 Deferred tax liabilities: Property, plant and equipment $ (21,312 ) $ (13,739 ) Intangible assets (47,887 ) (48,375 ) Loss on investment (4,790 ) (5,321 ) Other (3,660 ) (1,775 ) $ (77,649 ) $ (69,210 ) Net deferred tax liability before valuation allowance (11,645 ) (8,921 ) Valuation allowance (7,764 ) (7,427 ) Net deferred tax liability $ (19,409 ) $ (16,348 ) As of September 30, 2019 , the Company has $43,403 of state net operating loss carryforwards which expire beginning in 2019 through 2035. In certain non-U.S. jurisdictions, the Company has net operating loss carryforwards of $40,457 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, 2019 , the Company had unrecognized tax benefits of $794 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, 2019 and September 30, 2018 , the Company had accrued interest and penalties of $54 and $78 , 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) Balance as of September 30, 2016 $ 3,803 Additions based on tax positions related to prior years 63 Settlements (288 ) 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 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 jurisdiction's 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 jurisdiction's uncertain tax positions. The related accrued penalties and interest for uncertain tax positions decreased by $3,139 . During fiscal 2017 , the balance of unrecognized tax benefits decreased by $288 as a result of Tyco 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 decreased by $158 . 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 2016, 2017 and 2018 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, 2019 , the Company recorded a $635 provision 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. For the fiscal years ended September 30, 2018 and September 30, 2017 , the Company made no additional provision 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. As of September 30, 2019 , certain subsidiaries had approximately $52,404 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. For uncertain tax liabilities (including penalties and interest) arising in the periods prior to the Transactions that are resolved in a future period, the Company plans to seek repayment from Tyco under the terms of an investment agreement. Due to the statute of limitations expiring for the fiscal year ended September 30, 2019 , the Company has released all of those liabilities and the offsetting receivable due from Tyco. 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, 2019
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. Prior to fiscal 2018, earnings per share under the two-class method and the treasury method was the same, as net income allocated to participating securities was immaterial. 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, 2019 September 30, 2018 September 30, 2017 Numerator: Net income $ 139,051 $ 136,645 $ 84,639 Less: Undistributed earnings allocated to participating securities 3,726 2,456 241 Net income available to common shareholders $ 135,325 $ 134,189 $ 84,398 Denominator: Basic weighted average common shares outstanding 46,577 51,791 63,420 Effect of dilutive securities: Non-participating employee stock options (1) 1,200 2,298 3,134 Diluted weighted average common shares outstanding 47,777 54,089 66,554 Basic earnings per share $ 2.91 $ 2.59 $ 1.33 Diluted earnings per share $ 2.83 $ 2.48 $ 1.27 (1) Stock options to purchase approximately 0.4 million , 0.3 million , and 0.2 million shares of common stock were outstanding during the years ended September 30, 2019 , September 30, 2018 , and September 30, 2017 , 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, 2019
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, 2017 $ (10,445 ) $ (7,537 ) $ (17,982 ) Other comprehensive income (loss) before reclassifications 4,068 (2,853 ) 1,215 Amounts reclassified from accumulated other comprehensive loss 329 — 329 Net current period other comprehensive income (loss) 4,397 (2,853 ) 1,544 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 ) (1) Due to the adoption of ASU 2018-02. The following is a summary of the amounts reclassified from accumulated other comprehensive loss to net income : Fiscal Year Ended (in thousands) September 30, 2019 September 30, 2018 September 30, 2017 Amortization of defined benefit pension items: Amortization of net loss (included within other income, net) $ 101 $ 343 $ 1,303 Tax expense (25 ) (14 ) (497 ) Net reclassifications for the period $ 76 $ 329 $ 806 The estimated net actuarial loss for pension benefit plans that will be amortized from accumulated other comprehensive loss into net periodic benefit cost over the next fiscal year is expected to be approximately $888 .

Inventories, Net

Inventories, Net	12 Months Ended
	Sep. 30, 2019
Inventory Disclosure [Abstract]
Inventories, Net	12. INVENTORIES, NET A majority of the Company records inventory at the lower of cost or market (primarily last in, first out, or "LIFO"). Approximately 72% and 80% of the Company's inventories are valued at the lower of LIFO cost or market at September 30, 2019 and September 30, 2018 , respectively. (in thousands) September 30, 2019 September 30, 2018 Purchased materials and manufactured parts, net $ 52,742 $ 58,572 Work in process, net 21,424 21,769 Finished goods, net 151,924 141,412 Inventories, net $ 226,090 $ 221,753 Total inventories would be $3,138 and $26,340 higher than reported as of September 30, 2019 and September 30, 2018 , respectively, if the first-in, first-out method was used for all inventories. As of September 30, 2019 and September 30, 2018 , the excess and obsolete inventory reserve was $14,295 and $12,909

Property, Plant and Equipment

Property, Plant and Equipment	12 Months Ended
	Sep. 30, 2019
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	13. PROPERTY, PLANT AND EQUIPMENT As of September 30, 2019 and September 30, 2018 , property, plant and equipment at cost and accumulated depreciation were as follows: (in thousands) September 30, 2019 September 30, 2018 Land $ 19,897 $ 13,295 Buildings and related improvements 127,061 108,758 Machinery and equipment 318,421 262,078 Leasehold improvements 9,055 7,382 Software 24,835 30,502 Construction in progress 21,264 16,777 Property, plant and equipment 520,533 438,792 Accumulated depreciation (259,830 ) (225,684 ) Property, plant and equipment, net $ 260,703 $ 213,108 Depreciation expense for fiscal years ended September 30, 2019 , September 30, 2018 and September 30, 2017 totaled $39,471 , $34,786 and $32,320

Goodwill and Intangible Assets

Goodwill and Intangible Assets	12 Months Ended
	Sep. 30, 2019
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, 2017 $ 108,527 $ 39,189 $ 147,716 Goodwill divested during year — (2,626 ) (2,626 ) Goodwill acquired during year 813 — 813 Purchase price adjustments 24,527 — 24,527 Exchange rate effects $ (301 ) — $ (301 ) 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 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, 2019 and September 30, 2018 . 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, 2019 September 30, 2018 (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 $ 353,256 $ (171,777 ) $ 181,479 $ 330,295 $ (141,401 ) $ 188,894 Other 7 19,086 (7,761 ) 11,325 16,003 (5,861 ) 10,142 Total 372,342 (179,538 ) 192,804 346,298 (147,262 ) 199,036 Indefinite-lived Intangible Assets: Trade names 92,880 — 92,880 92,880 — 92,880 Total $ 465,222 $ (179,538 ) $ 285,684 $ 439,178 $ (147,262 ) $ 291,916 Amortization expense for the fiscal years ended September 30, 2019 , September 30, 2018 and September 30, 2017 was $32,876 , $32,104 and $22,407 , respectively. Expected amortization expense for intangible assets over the next five years and thereafter is as follows (in thousands) : 2020 $ 32,400 2021 32,320 2022 30,893 2023 30,575 2024 26,101 2025 and thereafter 40,515

Debt

Debt	12 Months Ended
	Sep. 30, 2019
Debt Disclosure [Abstract]
Debt	15. DEBT Debt as of September 30, 2019 and September 30, 2018 was as follows: (in thousands) September 30, 2019 September 30, 2018 First Lien Term Loan Facility due December 22, 2023 $ 851,361 $ 912,162 Deferred financing costs (6,569 ) (8,194 ) Other 525 279 Total debt $ 845,317 $ 904,247 Less: Current portion — 26,561 Long-term debt $ 845,317 $ 877,686 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. This voluntary prepayment resulted in the removal of all principal payment requirements until the contractual maturity of the debt in fiscal 2024. As of September 30, 2019 , future contractual maturities of long-term debt are as follows (in thousands) : 2020 $ — 2021 — 2022 — 2023 — 2024 852,120 2025 and thereafter $ — Term Loan Facilities — 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 million 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. For the fiscal year ended September 30, 2017 , the Company recorded a $9,805 loss on the extinguishment of the Initial First Lien Term Loan Facility and the Second Lien Term Loan Facility. 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 $425.0 million . 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 $375.0 million , 2) repay $42.0 million of outstanding loans under the ABL Credit Facility (as defined below) and 3) pay $5.8 million 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. The ABL Credit Facility has aggregate commitments of $325,000 and is guaranteed by AIH and the United States subsidiaries owned directly or indirectly by AII. AII's availability under the ABL Credit Facility was $301,882 and $315,119 as of September 30, 2019 and September 30, 2018 , 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 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, 2019 and September 30, 2018 , respectively. The company paid a fee of 0.50% on unutilized availability in fiscal 2016 and until December 22, 2016, and a fee of 0.375% for the period from December 22, 2016 through September 30, 2019 . The 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 include, without limitation: limitations on indebtedness, dividends and distributions, investments, 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 fiscal periods, changes in line of business and changes in charter documents. Additionally, if the availability under the ABL Credit Facility falls below certain levels, AII would subsequently be required to maintain a minimum fixed charge coverage ratio. AII has not been subject to the minimum fixed charge coverage ratio during any period subsequent to the establishment of the ABL Credit Facility.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Sep. 30, 2019
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 five 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 £ 45.0 million and £ 49.1 million as of September 30, 2019 and September 30, 2018 , respectively. There were no undesignated forward currency contracts as of September 30, 2017 . 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, 2019 and September 30, 2018 in accordance with the fair value hierarchy: September 30, 2019 September 30, 2018 (in thousands) Level 1 Level 2 Level 3 Level 1 Level 2 Level 3 Assets Cash equivalents $ 72,132 $ — $ — $ 28,175 $ — $ — Forward currency contracts — 3,420 — — — — Liabilities Forward currency contracts — $ — — — $ 1,857 — In addition to the recurring assets and liabilities measured at fair value, the Company had a contingent consideration liability of $650 as of September 30, 2018 measured at fair value related to the Cii acquisition. This was settled in the second quarter of fiscal 2019. The liability was valued using significant unobservable inputs (Level 3) based on the probability of achievement of first year earnings post acquisition. See Note 3, ''Acquisitions'' for further detail. 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, 2019 September 30, 2018 (in thousands) Carrying Value Fair Value Carrying Value Fair Value First Lien Term Loan Facility due December 22, 2023 $ 852,120 $ 853,543 $ 913,100 $ 916,113 Total debt $ 852,120 $ 853,543 $ 913,100 $ 916,113 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, 2019
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, 2019 , such obligations were $97,011 for fiscal 2020 , $1,826 for fiscal 2021 and $411 thereafter. These amounts represent open purchase orders for materials used in production. The Company leases certain facilities and equipment under operating leases. Total rental expense on all operating leases was $17,077 , $17,403 and $12,941 in fiscal 2019 , 2018 , and 2017 , respectively. At September 30, 2019 , minimum future operating lease payments for leases with terms in excess of one year are presented in the table below as follows (in thousands): 2020 $ 13,526 2021 11,592 2022 8,666 2023 6,362 2024 5,097 2025 and thereafter 6,938 Total $ 52,181 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 $2,424 and $6,755 as of September 30, 2019 and September 30, 2018 , respectively. As of September 30, 2019 , 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, Tyco and the Company agreed with a plaintiff to settle one Special Products claim that was to go to trial. 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. 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, 2019
Guarantees [Abstract]
Guarantees	18. GUARANTEES The Company has outstanding letters of credit totaling $10,501 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 $18,200 as of September 30, 2019 . 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, 2019
Related Party Transactions [Abstract]
Related Party Transactions	19. RELATED PARTY TRANSACTIONS In December 2010, 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. The fees related to the Consulting Agreement were paid quarterly, in advance and recorded as a component of selling, general and administrative expenses in the Company's consolidated statement of operations. In connection with the IPO on June 15, 2016, the Company entered into an agreement with CD&R to terminate the Consulting Agreement, including the ongoing consulting fees and paid CD&R a fee of $12,800 . The consulting and termination fees were $15,425 for the fiscal year ended September 30, 2016. 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 on September 30, 2019. 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 and 2017 to this customer were $6,000 and $8,300

Segment Information

Segment Information	12 Months Ended
	Sep. 30, 2019
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 sum of income (loss) from operations before income taxes, adjusted to exclude unallocated expenses, depreciation and amortization, interest expense, net, gain (loss) on extinguishment of debt, restructuring and impairments, stock-based compensation, certain legal matters, consulting fees, transaction costs, gain on sale of joint venture and other items, such as inventory reserves and adjustments, release of indemnified uncertain tax positions, the impact of foreign exchange gains or losses and the impact from the Fence and Sprinkler exit. 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, 2019 September 30, 2018 September 30, 2017 (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 $ 1,442,278 $ 1,215 $ 292,585 $ 1,365,067 $ 1,544 $ 255,260 $ 1,093,500 $ 1,283 $ 189,351 MP&S 474,260 — $ 70,040 470,072 81 $ 51,339 410,434 98 $ 63,687 Eliminations — (1,215 ) — (1,625 ) — (1,381 ) Consolidated operations $ 1,916,538 $ — $ 1,835,139 $ — $ 1,503,934 $ — Capital Expenditures Total Assets (in thousands) September 30, 2019 September 30, 2018 September 30, 2017 September 30, 2019 September 30, 2018 September 30, 2017 Electrical Raceway $ 19,856 $ 16,389 $ 13,037 $ 871,771 $ 751,024 $ 757,775 MP&S 13,934 14,267 8,212 252,708 291,164 306,229 Unallocated 1,070 7,845 3,873 312,516 281,872 151,088 Consolidated operations $ 34,860 $ 38,501 $ 25,122 $ 1,436,995 $ 1,324,060 $ 1,215,092 Presented below is a reconciliation of operating segment Adjusted EBITDA to Income before income taxes : Fiscal Year Ended (in thousands) September 30, 2019 September 30, 2018 September 30, 2017 Operating segment Adjusted EBITDA Electrical Raceway $ 292,585 $ 255,260 $ 189,351 MP&S 70,040 51,339 63,687 Total $ 362,625 $ 306,599 $ 253,038 Unallocated expenses (a) (38,217 ) (35,050 ) (25,430 ) Depreciation and amortization (72,347 ) (66,890 ) (54,727 ) Interest expense, net (50,473 ) (40,694 ) (26,598 ) Gain (loss) on extinguishment of debt — — (9,805 ) Restructuring charges (3,804 ) (1,849 ) (1,256 ) Stock-based compensation (11,798 ) (14,664 ) (12,788 ) Certain legal matters — 4,833 (7,551 ) Transaction costs (1,200 ) (9,314 ) (4,779 ) Gain on purchase of business 7,384 — — Gain on sale of a business — 27,575 — Gain on sale of joint venture — — 5,774 Other (b) (7,501 ) (4,194 ) 10,247 Income before income taxes $ 184,669 $ 166,352 $ 126,125 (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, release of indemnified uncertain tax positions and the impact of foreign exchange gains or losses. The Company's long-lived assets and net sales by geography were as follows: Long-lived assets Net sales (in thousands) September 30, 2019 September 30, 2018 September 30, 2017 September 30, 2019 September 30, 2018 September 30, 2017 United States $ 219,614 $ 201,101 $ 202,823 $ 1,689,194 $ 1,651,636 $ 1,367,907 Other Americas 147 138 164 33,485 43,013 37,908 Europe 43,207 11,090 9,306 142,279 90,915 55,181 Asia-Pacific 1,998 2,386 3,378 51,580 49,575 42,938 Total $ 264,966 $ 214,715 $ 215,671 $ 1,916,538 $ 1,835,139 $ 1,503,934 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, 2019 September 30, 2018 September 30, 2017 Metal Electrical Conduit and Fittings $ 546,533 $ 517,935 $ 349,239 Armored Cable and Fittings 360,494 336,388 323,070 PVC Electrical Conduit & Fittings 292,243 311,811 265,389 Other raceway products 243,008 198,933 155,802 Electrical Raceway 1,442,278 1,365,067 1,093,500 Mechanical Pipe 259,613 253,381 211,245 Other MP&S products 214,647 216,691 199,189 MP&S 474,260 470,072 410,434 Net sales $ 1,916,538 $ 1,835,139 $ 1,503,934 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, 2019, no single customer represented more than 10% of the Company's accounts receivable balance. As of September 30, 2018, one customer, Sonepar Management US, Inc., represented 11% of the Company's accounts receivable balance. As of November 28, 2018, all amounts outstanding as of September 30, 2018, that are due to be paid from Sonepar Management US, Inc., have been received. In fiscal 2017, no single customer accounted for more than 10% of sales or accounts receivable. Concentration of Employees — As of September 30, 2019 , approximately 27% of the Company's employees were represented by a union under a collective bargaining agreement. All unions are either located in the United States or Canada with no unions or Worker's Councils at any of the other locations abroad. Our Harvey, Illinois Special Metal Processing Facility agreement with the United Steelworkers Union, involving a bargaining unit of 15 employees, expired on November 11, 2018 and we successfully negotiated a new agreement which now expires in November 2022. Our Harvey, Illinois collective bargaining agreement with the United Steelworkers involves nearly 400 represented employees, is set expire in April 2020.

Quarterly Operating Results (Un

Quarterly Operating Results (Unaudited)	12 Months Ended
	Sep. 30, 2019
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, 2019 and September 30, 2018 . 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 28, 2018 March 29, 2019 June 28, 2019 September 30, 2019 (4) December 29, 2017 (1) March 30, 2018 (2) June 29, 2018 September 30, 2018 (3) Net sales $ 452,028 $ 469,309 $ 493,491 $ 501,710 $ 414,558 $ 445,000 $ 498,014 $ 477,567 Gross profit 110,256 117,088 126,134 143,722 96,867 109,157 120,329 111,731 Net income 26,949 29,555 36,550 45,997 27,189 42,558 34,199 32,699 Net income per share Basic (5) $ 0.56 $ 0.62 $ 0.77 $ 0.96 $ 0.43 $ 0.83 $ 0.73 $ 0.69 Diluted (5) 0.54 0.61 0.75 0.94 0.41 0.79 0.70 0.66 (1) Includes a $4,758 benefit due to the re-measurement of deferred tax liabilities as a result of the TCJA. See Note 9, "Income taxes" for additional information. (2) Includes $26,737 pre-tax gain on the sale of Flexhead. The Company recorded an additional working capital adjustment to the assets sold, which increased the gain by $838, in the quarter ending June 29, 2018. See Note 4, "Divestitures" for additional information. (3) Includes $7,171 pre-tax reversal of expense related to the Antidumping Duty Order for Malleable Iron Pipe Fittings. See Note 17, Commitments and contingencies" for additional information. (4) Includes a $7,384 gain on the acquisition of Cor-Tek. See Note 3, "Acquisitions" for additional information. (5) 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.

Schedule I - Condensed Financia

Schedule I - Condensed Financial Information	12 Months Ended
	Sep. 30, 2019
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, 2019 September 30, 2018 Assets Investment in subsidiary $ 232,936 $ 122,059 Total Assets 232,936 122,059 Liabilities and Equity Total Liabilities $ — $ — Equity: Common stock, $0.01 par value, 1,000,000,000 shares authorized, 46,955,163 and 47,079,645 shares issued and outstanding, respectively $ 471 $ 472 Treasury stock, held at cost, 260,900 and 260,900 shares, respectively (2,580 ) (2,580 ) Additional paid-in capital 477,139 457,978 Accumulated deficit (200,396 ) (317,373 ) Accumulated other comprehensive loss (41,698 ) (16,438 ) Total Equity 232,936 122,059 Total Liabilities and Equity $ 232,936 $ 122,059 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, 2019 September 30, 2018 September 30, 2017 Equity in net income of subsidiary $ 139,051 $ 136,645 $ 84,639 Net income 139,051 136,645 84,639 Other comprehensive (loss) income of subsidiary, net of tax (22,927 ) 1,544 7,968 Comprehensive income $ 116,124 $ 138,189 $ 92,607 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, 2019 September 30, 2018 September 30, 2017 Cash Flows from Operating Activities: Net cash provided by operating activities $ — $ — $ — Cash Flows from Investing Activities: Distribution received from subsidiary 24,419 411,775 13,938 Distribution paid to subsidiary (7,374 ) (20,110 ) (12,168 ) Net cash provided by investing activities 17,045 391,665 1,770 Cash Flows from Financing Activities: Issuance of common shares 7,374 20,110 12,168 Repurchase of common shares (24,419 ) (411,775 ) (13,938 ) Net cash used in financing activities (17,045 ) (391,665 ) (1,770 ) 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 $24,419 , $411,775 , and $13,938 from its subsidiaries for the years ended September 30, 2019 , September 30, 2018 and September 30, 2017 , 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, 2019
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: 2019 $ (1,762 ) (970 ) 124 $ (2,608 ) 2018 $ (1,239 ) (389 ) (134 ) $ (1,762 ) 2017 $ (1,006 ) (243 ) 10 $ (1,239 ) Deferred Tax Valuation Allowance: For the fiscal year ended: 2019 $ (7,427 ) (346 ) 9 $ (7,764 ) 2018 $ (9,512 ) 428 1,657 $ (7,427 ) 2017 $ (8,658 ) (1,164 ) 310 $ (9,512 )

Basis of Presentation and Sum_2

Basis of Presentation and Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Sep. 30, 2019
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 end on the last Friday in December, March and June.
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 production equipment 1 to 20 years Support and testing machinery and equipment 2 to 10 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. In the fourth quarter of 2017, the Company adopted Accounting Standards Update ("ASU") 2017-04, "Intangibles - Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment." This guidance eliminates the requirement to determine the fair value of individual assets and liabilities of a reporting unit to measure goodwill impairment. Under the amendments in the new guidance, the quantitative goodwill impairment testing is performed by comparing the fair value of the reporting unit with its carrying amount and recognizing an impairment charge for the amount by which the carrying amount exceeds the reporting unit's fair value. For fiscal 2019 and fiscal 2018, the Company performed a quantitative impairment assessment for goodwill. 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) a market approach using a transaction analysis. 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. Fair value under the guideline public company method is determined for each unit by applying market multiples for comparable public companies to the unit’s financial results. 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 unit by applying market multiples for comparable public companies to the unit’s financial results. Fair value under the transactional analysis 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 transactional analysis calculations are the assumptions used in determining the reporting unit's comparable public companies, comparable transactions and the selection of the market multiples. For fiscal 2017, the Company determined through qualitative assessment ("Step 0") that it was not “more likely than not” that the fair values of its reporting units were less than their carrying values; as a result, the Company was not required to perform the quantitative impairment test. The Company did not record any goodwill impairments in fiscal 2019, 2018 or 2017. 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 — 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.
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 2014-09 Revenue from Contracts with Customers and subsequent amendments The Accounting Standards Update ("ASU") provides guidance for revenue recognition. The update's core principle is that a company will recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. In doing so, companies will need to use more judgment and make more estimates than under current guidance. Examples of the use of judgments and estimates may include identifying performance obligations in the contract, estimating the amount of variable consideration to include in the transaction price and allocating the transaction price to each separate performance obligation. The update also requires more detailed disclosures to enable users of financial statements to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. The update provides for two transition methods to the new guidance: a full retrospective approach and a modified retrospective approach. The Company adopted the guidance in the first quarter of 2019 using the modified retrospective method. See Note 2, "Revenue from Contracts with Customers" for further detail. 2 2019 2018-02 Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income The ASU provided entities with the option to reclassify tax effects stranded in accumulated other comprehensive income as a result of the "H.R.1", also known as the "Tax Cuts and Jobs Act" ("TCJA") to retained earnings. The Company elected to adopt the guidance early in the quarter ended March 29, 2019. As a result of adoption of the ASU, the Company reclassified $2,333 of stranded tax benefits related to its pension plans out of Accumulated other comprehensive loss and into Accumulated deficit for the quarter ended March 29, 2019. The Company's policy is to release the tax effects as the related amounts in other comprehensive income are recognized in net income. 11 2019 2018-07 Improvements to Nonemployee Share-Based Payment Accounting. The ASU simplifies the accounting for share-based payments to nonemployees by aligning it with the accounting for share-based payments to employees, with certain exceptions. The Company elected to adopt the guidance early in the quarter ended June 28, 2019. There was no material impact to the consolidated financial statements as a result of the adoption of ASU 2018-07. 2019 2018-13 Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement The ASU amends ASC 820 to add, remove and clarify disclosure requirements related to fair value measurements. The Company elected to adopt the guidance early in the quarter ended June 28, 2019. There was no material impact to the consolidated financial statements as a result of the adoption of ASU 2018-13. 2019 A summary of guidance not yet adopted are as follows: Guidance not yet adopted ASU Description of ASU Impact to Atkore Effective 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 will adopt the new lease guidance in the first quarter of fiscal 2020 utilizing the modified retrospective transition method and will not restate comparative periods. We plan to elect the package of practical expedients permitted under the transition guidance within the ASU, which allows us to carry forward prior conclusions about lease identification, classification and initial direct costs for leases entered into prior to adoption. The new standard also provides practical expedients for an entity’s ongoing accounting. For leases with a term of 12 months or less, we plan to elect the short-term lease exemption, which allows us to not recognize right-of-use assets or lease liabilities for qualifying leases existing at transition and new leases we may enter into in the future. To assist with the ongoing accounting under ASC 842, we have implemented a new lease accounting system. The Company is continuing to assess the impact of the ASU on our consolidated financial statements, required disclosures, and changes to internal controls. Based on the work completed, the Company expects to recognize lease liabilities ranging from $42 million to $52 million, with corresponding right-of-use assets of the same amount. 2020 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. Under evaluation. 2021 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. Under evaluation. 2021

Basis of Presentation and Sum_3

Basis of Presentation and Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Sep. 30, 2019
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 production equipment 1 to 20 years Support and testing machinery and equipment 2 to 10 years Leasehold improvements Lesser of remaining term of the lease or useful life Software 2 to 10 years As of September 30, 2019 and September 30, 2018 , property, plant and equipment at cost and accumulated depreciation were as follows: (in thousands) September 30, 2019 September 30, 2018 Land $ 19,897 $ 13,295 Buildings and related improvements 127,061 108,758 Machinery and equipment 318,421 262,078 Leasehold improvements 9,055 7,382 Software 24,835 30,502 Construction in progress 21,264 16,777 Property, plant and equipment 520,533 438,792 Accumulated depreciation (259,830 ) (225,684 ) Property, plant and equipment, net $ 260,703 $ 213,108
Schedule of New Accounting Pronouncements and Changes in Accounting Principles	Adopted Guidance ASU Description of ASU Impact to Atkore Note Adoption Date 2014-09 Revenue from Contracts with Customers and subsequent amendments The Accounting Standards Update ("ASU") provides guidance for revenue recognition. The update's core principle is that a company will recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. In doing so, companies will need to use more judgment and make more estimates than under current guidance. Examples of the use of judgments and estimates may include identifying performance obligations in the contract, estimating the amount of variable consideration to include in the transaction price and allocating the transaction price to each separate performance obligation. The update also requires more detailed disclosures to enable users of financial statements to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. The update provides for two transition methods to the new guidance: a full retrospective approach and a modified retrospective approach. The Company adopted the guidance in the first quarter of 2019 using the modified retrospective method. See Note 2, "Revenue from Contracts with Customers" for further detail. 2 2019 2018-02 Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income The ASU provided entities with the option to reclassify tax effects stranded in accumulated other comprehensive income as a result of the "H.R.1", also known as the "Tax Cuts and Jobs Act" ("TCJA") to retained earnings. The Company elected to adopt the guidance early in the quarter ended March 29, 2019. As a result of adoption of the ASU, the Company reclassified $2,333 of stranded tax benefits related to its pension plans out of Accumulated other comprehensive loss and into Accumulated deficit for the quarter ended March 29, 2019. The Company's policy is to release the tax effects as the related amounts in other comprehensive income are recognized in net income. 11 2019 2018-07 Improvements to Nonemployee Share-Based Payment Accounting. The ASU simplifies the accounting for share-based payments to nonemployees by aligning it with the accounting for share-based payments to employees, with certain exceptions. The Company elected to adopt the guidance early in the quarter ended June 28, 2019. There was no material impact to the consolidated financial statements as a result of the adoption of ASU 2018-07. 2019 2018-13 Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement The ASU amends ASC 820 to add, remove and clarify disclosure requirements related to fair value measurements. The Company elected to adopt the guidance early in the quarter ended June 28, 2019. There was no material impact to the consolidated financial statements as a result of the adoption of ASU 2018-13. 2019 A summary of guidance not yet adopted are as follows: Guidance not yet adopted ASU Description of ASU Impact to Atkore Effective 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 will adopt the new lease guidance in the first quarter of fiscal 2020 utilizing the modified retrospective transition method and will not restate comparative periods. We plan to elect the package of practical expedients permitted under the transition guidance within the ASU, which allows us to carry forward prior conclusions about lease identification, classification and initial direct costs for leases entered into prior to adoption. The new standard also provides practical expedients for an entity’s ongoing accounting. For leases with a term of 12 months or less, we plan to elect the short-term lease exemption, which allows us to not recognize right-of-use assets or lease liabilities for qualifying leases existing at transition and new leases we may enter into in the future. To assist with the ongoing accounting under ASC 842, we have implemented a new lease accounting system. The Company is continuing to assess the impact of the ASU on our consolidated financial statements, required disclosures, and changes to internal controls. Based on the work completed, the Company expects to recognize lease liabilities ranging from $42 million to $52 million, with corresponding right-of-use assets of the same amount. 2020 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. Under evaluation. 2021 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. Under evaluation. 2021 September 30, 2019 was as follows: As of September 30, 2019 Balance Sheet As Reported Balances before adoption of ASC 606 Effect of Adoption Higher/(Lower) Accounts Receivable, net 315,353 271,900 43,453 Customer liabilities 44,983 1,530 43,453

Revenue from Contracts with C_2

Revenue from Contracts with Customers (Tables)	12 Months Ended
	Sep. 30, 2019
Revenue from Contract with Customer [Abstract]
Schedule of New Accounting Pronouncements and Changes in Accounting Principles	Adopted Guidance ASU Description of ASU Impact to Atkore Note Adoption Date 2014-09 Revenue from Contracts with Customers and subsequent amendments The Accounting Standards Update ("ASU") provides guidance for revenue recognition. The update's core principle is that a company will recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. In doing so, companies will need to use more judgment and make more estimates than under current guidance. Examples of the use of judgments and estimates may include identifying performance obligations in the contract, estimating the amount of variable consideration to include in the transaction price and allocating the transaction price to each separate performance obligation. The update also requires more detailed disclosures to enable users of financial statements to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. The update provides for two transition methods to the new guidance: a full retrospective approach and a modified retrospective approach. The Company adopted the guidance in the first quarter of 2019 using the modified retrospective method. See Note 2, "Revenue from Contracts with Customers" for further detail. 2 2019 2018-02 Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income The ASU provided entities with the option to reclassify tax effects stranded in accumulated other comprehensive income as a result of the "H.R.1", also known as the "Tax Cuts and Jobs Act" ("TCJA") to retained earnings. The Company elected to adopt the guidance early in the quarter ended March 29, 2019. As a result of adoption of the ASU, the Company reclassified $2,333 of stranded tax benefits related to its pension plans out of Accumulated other comprehensive loss and into Accumulated deficit for the quarter ended March 29, 2019. The Company's policy is to release the tax effects as the related amounts in other comprehensive income are recognized in net income. 11 2019 2018-07 Improvements to Nonemployee Share-Based Payment Accounting. The ASU simplifies the accounting for share-based payments to nonemployees by aligning it with the accounting for share-based payments to employees, with certain exceptions. The Company elected to adopt the guidance early in the quarter ended June 28, 2019. There was no material impact to the consolidated financial statements as a result of the adoption of ASU 2018-07. 2019 2018-13 Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement The ASU amends ASC 820 to add, remove and clarify disclosure requirements related to fair value measurements. The Company elected to adopt the guidance early in the quarter ended June 28, 2019. There was no material impact to the consolidated financial statements as a result of the adoption of ASU 2018-13. 2019 A summary of guidance not yet adopted are as follows: Guidance not yet adopted ASU Description of ASU Impact to Atkore Effective 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 will adopt the new lease guidance in the first quarter of fiscal 2020 utilizing the modified retrospective transition method and will not restate comparative periods. We plan to elect the package of practical expedients permitted under the transition guidance within the ASU, which allows us to carry forward prior conclusions about lease identification, classification and initial direct costs for leases entered into prior to adoption. The new standard also provides practical expedients for an entity’s ongoing accounting. For leases with a term of 12 months or less, we plan to elect the short-term lease exemption, which allows us to not recognize right-of-use assets or lease liabilities for qualifying leases existing at transition and new leases we may enter into in the future. To assist with the ongoing accounting under ASC 842, we have implemented a new lease accounting system. The Company is continuing to assess the impact of the ASU on our consolidated financial statements, required disclosures, and changes to internal controls. Based on the work completed, the Company expects to recognize lease liabilities ranging from $42 million to $52 million, with corresponding right-of-use assets of the same amount. 2020 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. Under evaluation. 2021 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. Under evaluation. 2021 September 30, 2019 was as follows: As of September 30, 2019 Balance Sheet As Reported Balances before adoption of ASC 606 Effect of Adoption Higher/(Lower) Accounts Receivable, net 315,353 271,900 43,453 Customer liabilities 44,983 1,530 43,453

Acquisitions (Tables)

Acquisitions (Tables)	12 Months Ended
	Sep. 30, 2019
Business Combinations [Abstract]
Schedule of Net 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 2017: (in thousands) Calpipe Other Total Fair value of consideration transferred: Cash consideration $ 110,155 $ 87,649 $ 197,804 Working capital adjustment 120 — 120 Purchase price payable 2,278 — 2,278 Settlement of pre-existing relationship (382 ) — (382 ) Total consideration transferred 112,171 87,649 199,820 Fair value of assets acquired and liabilities assumed: Cash 5,051 8,830 13,881 Accounts receivable 10,369 7,589 17,958 Inventories 18,360 7,221 25,581 Intangible assets 54,860 40,100 94,960 Fixed assets 3,245 11,242 14,487 Accounts payable (1,601 ) (1,550 ) (3,151 ) Other (8,342 ) (11,558 ) (19,900 ) Net assets acquired 81,942 61,874 143,816 Excess purchase price attributed to goodwill acquired $ 30,229 $ 25,775 $ 56,004 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 Fair Value of Amortizable Intangible Assets	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 The following table summarizes the fair value of intangible assets acquired in fiscal 2017 as of the acquisition dates: Calpipe Other ($ in thousands) Fair Value Weighted Average Useful Life (Years) Fair Value Weighted Average Useful Life (Years) Customer relationships $ 50,680 10.0 $ 37,341 10.0 Other 4,180 8.5 2,759 8.1 Total intangible assets $ 54,860 9.9 $ 40,100 9.9
Business Acquisition, Pro Forma Information	The following table presents unaudited pro forma results of operations for the fiscal year ended September 30, 2017 as if the Calpipe acquisition had occurred as of the first day of the fiscal 2017 period: Fiscal Year Ended (in thousands) September 30, 2017 Pro forma net sales $ 1,575,801 Pro forma net income 91,362

Divestitures (Tables)

Divestitures (Tables)	12 Months Ended
	Sep. 30, 2019
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, 2019
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, 2019 September 30, 2018 September 30, 2017 Service cost $ — $ — $ 2,049 Interest cost 4,665 4,098 3,793 Expected return on plan assets (6,371 ) (6,415 ) (6,601 ) Amortization of actuarial loss 101 343 1,303 Net periodic (benefit) cost $ (1,605 ) $ (1,974 ) $ 544 The weighted-average assumptions used to determine net periodic pension (benefit) cost during the period were as follow: September 30, 2019 September 30, 2018 September 30, 2017 Discount rate 4.2 % 3.7 % 3.5 % Expected return on plan assets 6.3 % 6.4 % 7.0 % 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, 2017 $ 128,852 Interest cost 4,098 Actuarial gain (7,130 ) Benefits and administrative expenses paid (4,863 ) 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 Change in plan assets: Balance as of September 30, 2017 $ 103,613 Actual return on plan assets 4,701 Employer contributions 1,003 Benefits and administrative expenses paid (4,863 ) 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 Funded status: Funded status as of September 30, 2018 $ (16,503 ) Funded status as of September 30, 2019 $ (34,509 )
Schedule of Amounts Recognized in Balance Sheet	(in thousands) September 30, 2019 September 30, 2018 Amounts recognized in the consolidated balance sheets consist of: Pension assets $ — $ 572 Pension liabilities (34,509 ) (17,075 ) Net amount recognized $ (34,509 ) $ (16,503 ) Amounts recognized in accumulated other comprehensive loss (before income taxes) consist of: Net actuarial loss $ (33,035 ) $ (12,344 ) Total loss recognized $ (33,035 ) $ (12,344 ) Weighted-average assumptions used to determine pension benefit obligations at year end: Discount rate 3.1 % 4.2 % 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, 2019 September 30, 2018 Accumulated benefit obligation $ 137,959 $ 105,414 Fair value of plan assets 103,451 88,339 The following table summarizes the defined benefit pension plans with projected benefit obligations in excess of plan assets: (in thousands) September 30, 2019 September 30, 2018 Projected benefit obligation $ 137,959 $ 105,414 Fair value of plan assets 103,451 88,339
Schedule of Allocation of Plan Assets	The Company's asset allocations are presented in the table below: (in thousands) September 30, 2019 September 30, 2018 Level 1 Level 2 Total Level 1 Level 2 Total United States equity securities $ 32,545 $ 6,429 $ 38,974 $ 32,447 $ 6,354 $ 38,801 Non-U.S. equity securities 18,182 2,543 20,725 20,164 2,599 22,763 Fixed income securities 35,214 6,815 42,029 34,111 6,678 40,789 Cash and cash equivalents 1,723 — 1,723 2,101 — 2,101 Total $ 87,664 $ 15,787 $ 103,451 $ 88,823 $ 15,631 $ 104,454 Pension plans have the following weighted-average asset allocations: Asset Category: September 30, 2019 September 30, 2018 Equity securities 58% 59% Debt securities 40% 39% Cash and cash equivalents 2% 2% Total 100% 100%
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) 2020 $ 5,841 2021 6,159 2022 6,442 2023 6,670 2024 6,903 2025 to 2029 37,071

Stock Incentive Plan (Tables)

Stock Incentive Plan (Tables)	12 Months Ended
	Sep. 30, 2019
Share-based Payment Arrangement [Abstract]
Schedule of Assumptions	The assumptions used in the Monte-Carlo method to value the performance share awards granted and modified during the fiscal year ended September 30, 2019 were as follows: September 30, 2019 September 30, 2018 September 30, 2017 Expected dividend yield — % — % — % Range of expected volatility 20.69 - 74.26 17.21 - 79.15 17.55 - 75.55 Risk free interest rates 2.84 % 1.83% - 2.54% 1.35 % Expected life 3 years 1 -3 years 3 years Fair value $ 18.84 $ 13.90 - 26.68 $ 29.53 The assumptions used in the Black-Scholes option pricing model to value the options granted and modified were as follows: Fiscal Year Ended September 30, 2019 September 30, 2018 September 30, 2017 Expected dividend yield — % — % — % Expected volatility 45 % 40 % 40 % Range of risk-free interest rates 2.94% - 3.00% 2.16% - 2.38% 1.95 % Range of expected option lives 6 to 10 years 1 to 6 years 6 years
Schedule of Stock Option Activity	Stock option activity for the period September 30, 2016 to September 30, 2019 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, 2016 6,664 $ 7.76 Granted 171 21.45 $ 8.84 Exercised (1,629 ) 7.47 $ 25,757 Forfeited (48 ) 9.02 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 5.72 Exercisable as of September 30, 2019 1,301 $ 9.46 $ 27,185 4.63
Schedule of Nonvested Share Activity	Changes to the Company's nonvested RSU awards for the year ended September 30, 2019 were as follows: Shares Weighted-average grant-date fair value Nonvested as of September 30, 2016 — $ — Granted 443 19.68 Forfeited (8 ) 21.45 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 Changes to the Company's non-vested PSU awards for the year ended September 30, 2019 were as follows: Shares Weighted-average grant-date fair value Nonvested as of September 30, 2016 — $ — Granted 184 23.87 Forfeited (3 ) 23.87 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

Restructuring Charges (Tables)

Restructuring Charges (Tables)	12 Months Ended
	Sep. 30, 2019
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 (a) Severance (a) Other (a) Severance Total Balance as of September 30, 2016 $ 841 $ — $ — $ 539 $ — $ 1,380 Charges 527 439 422 63 71 1,522 Utilization (917 ) (209 ) (166 ) (556 ) (71 ) (1,919 ) Reversals — (230 ) — (36 ) — (266 ) Exchange rate effects (2 ) — 22 — — 20 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 (a) 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, 2019 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, 2019 September 30, 2018 September 30, 2017 Total restructuring charges, net $ 3,804 $ 1,849 $ 1,256

Other Income, Net (Tables)

Other Income, Net (Tables)	12 Months Ended
	Sep. 30, 2019
Other Income, Nonoperating [Abstract]
Other Income, Net	Other income, net consisted of the following: Fiscal Year Ended (in thousands) September 30, 2019 September 30, 2018 September 30, 2017 Gain on purchase of business $ (7,384 ) $ — $ — Gain on sale of a business — (27,575 ) — Gain on sale of joint venture — — (5,774 ) Undesignated foreign currency derivative instruments (5,384 ) (121 ) 2,741 Foreign exchange loss (gain) on intercompany loans 2,975 1,500 (2,038 ) Debt modification costs — 892 — Pension-related benefits (1,606 ) (1,975 ) (1,504 ) Other (79 ) (69 ) — Other income, net $ (11,478 ) $ (27,348 ) $ (6,575 )

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Sep. 30, 2019
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, 2019 , September 30, 2018 and September 30, 2017 consisted of the following: (in thousands) September 30, 2019 September 30, 2018 September 30, 2017 Components of income before income taxes: United States $ 170,780 $ 155,453 $ 116,580 Non-U.S 13,889 10,899 9,545 Income before income taxes $ 184,669 $ 166,352 $ 126,125 Income tax expense: Current: United States: Federal $ 31,431 $ 26,586 $ 33,127 State 9,800 8,662 4,246 Non-U.S: 5,183 3,467 3,175 Current income tax expense $ 46,414 $ 38,715 $ 40,548 Deferred: United States: Federal $ 1,123 $ (7,149 ) $ 224 State (340 ) (1,793 ) 469 Non-U.S: (1,579 ) (66 ) 245 Deferred income (benefit) tax expense (796 ) (9,008 ) 938 Income tax expense $ 45,618 $ 29,707 $ 41,486
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, 2019 September 30, 2018 September 30, 2017 Statutory federal tax 21 % 25 % 35 % Adjustments to reconcile to the effective income tax rate: State income taxes 4 % 3 % 3 % Change in tax rates — % (3 )% — % Domestic manufacturing deduction — % (2 )% (2 )% Indemnified uncertain tax benefits — % (2 )% — % Stock-based compensation (1 )% (3 )% (3 )% Other 1 % — % — % Effective income tax rate 25 % 18 % 33 %
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, 2019 September 30, 2018 Deferred tax assets: Accrued liabilities and reserves $ 32,664 $ 32,099 Tax loss and credit carryforwards 13,296 12,435 Postretirement benefits 10,315 4,317 Inventory 8,813 11,220 Other 916 218 $ 66,004 $ 60,289 Deferred tax liabilities: Property, plant and equipment $ (21,312 ) $ (13,739 ) Intangible assets (47,887 ) (48,375 ) Loss on investment (4,790 ) (5,321 ) Other (3,660 ) (1,775 ) $ (77,649 ) $ (69,210 ) Net deferred tax liability before valuation allowance (11,645 ) (8,921 ) Valuation allowance (7,764 ) (7,427 ) Net deferred tax liability $ (19,409 ) $ (16,348 )
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) Balance as of September 30, 2016 $ 3,803 Additions based on tax positions related to prior years 63 Settlements (288 ) 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
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 2016, 2017 and 2018

Earnings Per Share (Tables)

Earnings Per Share (Tables)	12 Months Ended
	Sep. 30, 2019
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, 2019 September 30, 2018 September 30, 2017 Numerator: Net income $ 139,051 $ 136,645 $ 84,639 Less: Undistributed earnings allocated to participating securities 3,726 2,456 241 Net income available to common shareholders $ 135,325 $ 134,189 $ 84,398 Denominator: Basic weighted average common shares outstanding 46,577 51,791 63,420 Effect of dilutive securities: Non-participating employee stock options (1) 1,200 2,298 3,134 Diluted weighted average common shares outstanding 47,777 54,089 66,554 Basic earnings per share $ 2.91 $ 2.59 $ 1.33 Diluted earnings per share $ 2.83 $ 2.48 $ 1.27 (1) Stock options to purchase approximately 0.4 million , 0.3 million , and 0.2 million shares of common stock were outstanding during the years ended September 30, 2019 , September 30, 2018 , and September 30, 2017 , 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, 2019
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, 2017 $ (10,445 ) $ (7,537 ) $ (17,982 ) Other comprehensive income (loss) before reclassifications 4,068 (2,853 ) 1,215 Amounts reclassified from accumulated other comprehensive loss 329 — 329 Net current period other comprehensive income (loss) 4,397 (2,853 ) 1,544 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 ) (1) Due to the adoption of ASU 2018-02.
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, 2019 September 30, 2018 September 30, 2017 Amortization of defined benefit pension items: Amortization of net loss (included within other income, net) $ 101 $ 343 $ 1,303 Tax expense (25 ) (14 ) (497 ) Net reclassifications for the period $ 76 $ 329 $ 806

Inventories, Net (Tables)

Inventories, Net (Tables)	12 Months Ended
	Sep. 30, 2019
Inventory Disclosure [Abstract]
Schedule of Inventory, Net	Approximately 72% and 80% of the Company's inventories are valued at the lower of LIFO cost or market at September 30, 2019 and September 30, 2018 , respectively. (in thousands) September 30, 2019 September 30, 2018 Purchased materials and manufactured parts, net $ 52,742 $ 58,572 Work in process, net 21,424 21,769 Finished goods, net 151,924 141,412 Inventories, net $ 226,090 $ 221,753

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	12 Months Ended
	Sep. 30, 2019
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 production equipment 1 to 20 years Support and testing machinery and equipment 2 to 10 years Leasehold improvements Lesser of remaining term of the lease or useful life Software 2 to 10 years As of September 30, 2019 and September 30, 2018 , property, plant and equipment at cost and accumulated depreciation were as follows: (in thousands) September 30, 2019 September 30, 2018 Land $ 19,897 $ 13,295 Buildings and related improvements 127,061 108,758 Machinery and equipment 318,421 262,078 Leasehold improvements 9,055 7,382 Software 24,835 30,502 Construction in progress 21,264 16,777 Property, plant and equipment 520,533 438,792 Accumulated depreciation (259,830 ) (225,684 ) Property, plant and equipment, net $ 260,703 $ 213,108

Goodwill and Intangible Assets

Goodwill and Intangible Assets (Tables)	12 Months Ended
	Sep. 30, 2019
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, 2017 $ 108,527 $ 39,189 $ 147,716 Goodwill divested during year — (2,626 ) (2,626 ) Goodwill acquired during year 813 — 813 Purchase price adjustments 24,527 — 24,527 Exchange rate effects $ (301 ) — $ (301 ) 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
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, 2019 September 30, 2018 (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 $ 353,256 $ (171,777 ) $ 181,479 $ 330,295 $ (141,401 ) $ 188,894 Other 7 19,086 (7,761 ) 11,325 16,003 (5,861 ) 10,142 Total 372,342 (179,538 ) 192,804 346,298 (147,262 ) 199,036 Indefinite-lived Intangible Assets: Trade names 92,880 — 92,880 92,880 — 92,880 Total $ 465,222 $ (179,538 ) $ 285,684 $ 439,178 $ (147,262 ) $ 291,916
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, 2019 September 30, 2018 (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 $ 353,256 $ (171,777 ) $ 181,479 $ 330,295 $ (141,401 ) $ 188,894 Other 7 19,086 (7,761 ) 11,325 16,003 (5,861 ) 10,142 Total 372,342 (179,538 ) 192,804 346,298 (147,262 ) 199,036 Indefinite-lived Intangible Assets: Trade names 92,880 — 92,880 92,880 — 92,880 Total $ 465,222 $ (179,538 ) $ 285,684 $ 439,178 $ (147,262 ) $ 291,916
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) : 2020 $ 32,400 2021 32,320 2022 30,893 2023 30,575 2024 26,101 2025 and thereafter 40,515

Debt (Tables)

Debt (Tables)	12 Months Ended
	Sep. 30, 2019
Debt Disclosure [Abstract]
Schedule of Debt	Debt as of September 30, 2019 and September 30, 2018 was as follows: (in thousands) September 30, 2019 September 30, 2018 First Lien Term Loan Facility due December 22, 2023 $ 851,361 $ 912,162 Deferred financing costs (6,569 ) (8,194 ) Other 525 279 Total debt $ 845,317 $ 904,247 Less: Current portion — 26,561 Long-term debt $ 845,317 $ 877,686
Contractual Obligation, Fiscal Year Maturity Schedule	As of September 30, 2019 , future contractual maturities of long-term debt are as follows (in thousands) : 2020 $ — 2021 — 2022 — 2023 — 2024 852,120 2025 and thereafter $ —

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Sep. 30, 2019
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, 2019 and September 30, 2018 in accordance with the fair value hierarchy: September 30, 2019 September 30, 2018 (in thousands) Level 1 Level 2 Level 3 Level 1 Level 2 Level 3 Assets Cash equivalents $ 72,132 $ — $ — $ 28,175 $ — $ — Forward currency contracts — 3,420 — — — — Liabilities Forward currency contracts — $ — — — $ 1,857 —
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, 2019 September 30, 2018 (in thousands) Carrying Value Fair Value Carrying Value Fair Value First Lien Term Loan Facility due December 22, 2023 $ 852,120 $ 853,543 $ 913,100 $ 916,113 Total debt $ 852,120 $ 853,543 $ 913,100 $ 916,113

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Sep. 30, 2019
Commitments and Contingencies Disclosure [Abstract]
Schedule of Minimum Future Operating Lease Payments	At September 30, 2019 , minimum future operating lease payments for leases with terms in excess of one year are presented in the table below as follows (in thousands): 2020 $ 13,526 2021 11,592 2022 8,666 2023 6,362 2024 5,097 2025 and thereafter 6,938 Total $ 52,181

Segment Information (Tables)

Segment Information (Tables)	12 Months Ended
	Sep. 30, 2019
Segment Reporting [Abstract]
Schedule of Segment Information	Presented below is a reconciliation of operating segment Adjusted EBITDA to Income before income taxes : Fiscal Year Ended (in thousands) September 30, 2019 September 30, 2018 September 30, 2017 Operating segment Adjusted EBITDA Electrical Raceway $ 292,585 $ 255,260 $ 189,351 MP&S 70,040 51,339 63,687 Total $ 362,625 $ 306,599 $ 253,038 Unallocated expenses (a) (38,217 ) (35,050 ) (25,430 ) Depreciation and amortization (72,347 ) (66,890 ) (54,727 ) Interest expense, net (50,473 ) (40,694 ) (26,598 ) Gain (loss) on extinguishment of debt — — (9,805 ) Restructuring charges (3,804 ) (1,849 ) (1,256 ) Stock-based compensation (11,798 ) (14,664 ) (12,788 ) Certain legal matters — 4,833 (7,551 ) Transaction costs (1,200 ) (9,314 ) (4,779 ) Gain on purchase of business 7,384 — — Gain on sale of a business — 27,575 — Gain on sale of joint venture — — 5,774 Other (b) (7,501 ) (4,194 ) 10,247 Income before income taxes $ 184,669 $ 166,352 $ 126,125 (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, release of indemnified uncertain tax positions and the impact of foreign exchange gains or losses. Fiscal year ended September 30, 2019 September 30, 2018 September 30, 2017 (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 $ 1,442,278 $ 1,215 $ 292,585 $ 1,365,067 $ 1,544 $ 255,260 $ 1,093,500 $ 1,283 $ 189,351 MP&S 474,260 — $ 70,040 470,072 81 $ 51,339 410,434 98 $ 63,687 Eliminations — (1,215 ) — (1,625 ) — (1,381 ) Consolidated operations $ 1,916,538 $ — $ 1,835,139 $ — $ 1,503,934 $ — Capital Expenditures Total Assets (in thousands) September 30, 2019 September 30, 2018 September 30, 2017 September 30, 2019 September 30, 2018 September 30, 2017 Electrical Raceway $ 19,856 $ 16,389 $ 13,037 $ 871,771 $ 751,024 $ 757,775 MP&S 13,934 14,267 8,212 252,708 291,164 306,229 Unallocated 1,070 7,845 3,873 312,516 281,872 151,088 Consolidated operations $ 34,860 $ 38,501 $ 25,122 $ 1,436,995 $ 1,324,060 $ 1,215,092
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, 2019 September 30, 2018 September 30, 2017 September 30, 2019 September 30, 2018 September 30, 2017 United States $ 219,614 $ 201,101 $ 202,823 $ 1,689,194 $ 1,651,636 $ 1,367,907 Other Americas 147 138 164 33,485 43,013 37,908 Europe 43,207 11,090 9,306 142,279 90,915 55,181 Asia-Pacific 1,998 2,386 3,378 51,580 49,575 42,938 Total $ 264,966 $ 214,715 $ 215,671 $ 1,916,538 $ 1,835,139 $ 1,503,934
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, 2019 September 30, 2018 September 30, 2017 Metal Electrical Conduit and Fittings $ 546,533 $ 517,935 $ 349,239 Armored Cable and Fittings 360,494 336,388 323,070 PVC Electrical Conduit & Fittings 292,243 311,811 265,389 Other raceway products 243,008 198,933 155,802 Electrical Raceway 1,442,278 1,365,067 1,093,500 Mechanical Pipe 259,613 253,381 211,245 Other MP&S products 214,647 216,691 199,189 MP&S 474,260 470,072 410,434 Net sales $ 1,916,538 $ 1,835,139 $ 1,503,934

Quarterly Operating Results (Ta

Quarterly Operating Results (Tables)	12 Months Ended
	Sep. 30, 2019
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, 2019 and September 30, 2018 . 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 28, 2018 March 29, 2019 June 28, 2019 September 30, 2019 (4) December 29, 2017 (1) March 30, 2018 (2) June 29, 2018 September 30, 2018 (3) Net sales $ 452,028 $ 469,309 $ 493,491 $ 501,710 $ 414,558 $ 445,000 $ 498,014 $ 477,567 Gross profit 110,256 117,088 126,134 143,722 96,867 109,157 120,329 111,731 Net income 26,949 29,555 36,550 45,997 27,189 42,558 34,199 32,699 Net income per share Basic (5) $ 0.56 $ 0.62 $ 0.77 $ 0.96 $ 0.43 $ 0.83 $ 0.73 $ 0.69 Diluted (5) 0.54 0.61 0.75 0.94 0.41 0.79 0.70 0.66 (1) Includes a $4,758 benefit due to the re-measurement of deferred tax liabilities as a result of the TCJA. See Note 9, "Income taxes" for additional information. (2) Includes $26,737 pre-tax gain on the sale of Flexhead. The Company recorded an additional working capital adjustment to the assets sold, which increased the gain by $838, in the quarter ending June 29, 2018. See Note 4, "Divestitures" for additional information. (3) Includes $7,171 pre-tax reversal of expense related to the Antidumping Duty Order for Malleable Iron Pipe Fittings. See Note 17, Commitments and contingencies" for additional information. (4) Includes a $7,384 gain on the acquisition of Cor-Tek. See Note 3, "Acquisitions" for additional information. (5) 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 - The Transactions (Details) - USD ($) $ in Thousands, shares in Millions	Apr. 09, 2014	Mar. 06, 2014	Dec. 22, 2010
Debt Instrument [Line Items]
Dividend rate, percentage (%)			12.00%
Payment to redeem shares	$ 250,000	$ 250,000
Atkore International | CD&R
Debt Instrument [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%
Common Stock
Debt Instrument [Line Items]
Non-binding letter of intent to acquire common stock (shares)		40.3
Expense related to share redemption	$ 2,000	$ 2,000

Basis of Presentation and Sum_5

Basis of Presentation and Summary of Significant Accounting Policies - Initial Public Offering (Details) - $ / shares	Jun. 09, 2016	Sep. 30, 2017
Subsidiary, Sale of Stock [Line Items]
Shares of common stock issued (shares)		19,550,000
IPO
Subsidiary, Sale of Stock [Line Items]
Public offering price (in dollars per share)	$ 16
Shares of common stock issued (shares)	12,000,000

Basis of Presentation and Sum_6

Basis of Presentation and Summary of Significant Accounting Policies - Share Repurchase Program (Details) - USD ($) $ / shares in Units, shares in Millions	Jan. 22, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Feb. 05, 2019	Aug. 31, 2017
Related Party Transaction [Line Items]
Common stock, par value (in dollars per share)		$ 0.01	$ 0.01
Repurchase of common stock		$ 24,419,000	$ 411,775,000	$ 13,938,000
Stock repurchase program, authorized amount					$ 50,000,000	$ 75,000,000
Remaining authorized repurchase amount		$ 50,000,000		$ 24,389,000
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	$ 375,000,000
CD&R Allied Holdings, L.P. | Common Stock
Related Party Transaction [Line Items]
Stock repurchase program, number of shares authorized to be repurchased (shares)	17.2

Basis of Presentation and Sum_7

Basis of Presentation and Summary of Significant Accounting Policies - Property, Plant and Equipment (Details)	12 Months Ended
	Sep. 30, 2019
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 | Support and testing machinery and equipment
Property, Plant and Equipment [Line Items]
Estimated useful lives (years)	2 years
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 | Support and testing machinery and equipment
Property, Plant and Equipment [Line Items]
Estimated useful lives (years)	10 years
Maximum | Software
Property, Plant and Equipment [Line Items]
Estimated useful lives (years)	10 years

Basis of Presentation and Sum_8

Basis of Presentation and Summary of Significant Accounting Policies - Goodwill and Indefinite-Lived Intangible Asset Impairments (Details)	12 Months Ended
	Sep. 30, 2019
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

Basis of Presentation and Sum_9

Basis of Presentation and Summary of Significant Accounting Policies Basis of Presentation and Summary of Significant Accounting Policies - Recent Accounting Pronouncements (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
		Mar. 29, 2019	Sep. 30, 2019		Oct. 01, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification of stranded tax benefits	[1]		$ 0
Accumulated Deficit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification of stranded tax benefits		$ 2,333	$ 2,333	[1]
Forecast | Minimum | Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, liability					$ 42,000
Operating lease, right-of-use asset					42,000
Forecast | Maximum | Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, liability					52,000
Operating lease, right-of-use asset					$ 52,000
[1]	(1) Due to the adoption of ASU 2018-02.

Revenue from Contracts with C_3

Revenue from Contracts with Customers (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts Receivable, net	$ 315,353	$ 265,147
Customer liabilities	44,983	$ 3,377
Balances before adoption of ASC 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts Receivable, net	271,900
Customer liabilities	1,530
Effect of Adoption Higher/(Lower)
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts Receivable, net	43,453
Customer liabilities	$ 43,453

Acquisitions - Narrative (Detai

Acquisitions - Narrative (Details) $ in Thousands	Aug. 21, 2019USD ($)	Jun. 03, 2019USD ($)	Oct. 01, 2018USD ($)countries	Jan. 08, 2018USD ($)	Sep. 30, 2019USD ($)	Sep. 30, 2019USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 01, 2017countries
Business Acquisition [Line Items]
Bargain purchase gain						$ 7,384	$ 0	$ 0
Acquisition-related expenses							558	1,676
Purchase price adjustments							24,527
Intangibles, purchase accounting adjustments							18,793
Property, plant, and equipment, purchase accounting adjustments							2,536
Deferred tax liabilities, purchase accounting adjustments							$ 8,270
Amortization period for tax-deductible goodwill						15 years	15 years
Goodwill					$ 186,231	$ 186,231	$ 170,129	147,716
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
Number of countries in which entity operates | countries			45
Cash consideration						58,728
Goodwill					9,629	9,629
Intangible assets					$ 12,621	12,621
Communications Integrators, Inc
Business Acquisition [Line Items]
Cash consideration, net of cash acquired				$ 3,997
Flexicon Limited
Business Acquisition [Line Items]
Number of countries in which entity operates | countries									55
Calpipe Industries
Business Acquisition [Line Items]
Cash consideration								110,155
Goodwill								30,229
Intangible assets								54,860
Working capital adjustment								(120)
Fair Value Adjustment to Inventory
Business Acquisition [Line Items]
Pro forma adjustment to reflect results of operations								$ 2,159
Selling, General and Administrative Expenses
Business Acquisition [Line Items]
Acquisition-related expenses						$ 1,140

Acquisitions - Net Assets Acqui

Acquisitions - Net Assets Acquired and Liabilites Assumed (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2017	Sep. 30, 2018
Fair value of assets acquired and liabilities assumed:
Excess purchase price attributed to goodwill acquired	$ 186,231	$ 147,716	$ 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
Calpipe Industries
Fair value of consideration transferred:
Cash consideration		110,155
Working capital adjustment		120
Purchase price payable		2,278
Settlement of pre-existing relationship		(382)
Total consideration transferred		112,171
Fair value of assets acquired and liabilities assumed:
Cash		5,051
Accounts receivable		10,369
Inventories		18,360
Intangible assets		54,860
Fixed assets		3,245
Accounts payable		(1,601)
Other		(8,342)
Net assets acquired		81,942
Excess purchase price attributed to goodwill acquired		30,229
Other Acquisitions
Fair value of consideration transferred:
Cash consideration	41,641	87,649
Working capital adjustment		0
Purchase price payable		0
Other liability consideration	1,400
Settlement of pre-existing relationship		0
Total consideration transferred	43,041	87,649
Fair value of assets acquired and liabilities assumed:
Cash	1,541	8,830
Accounts receivable	8,217	7,589
Inventories	7,494	7,221
Intangible assets	16,400	40,100
Fixed assets	19,298	11,242
Accounts payable	(7,608)	(1,550)
Gain on purchase of business	(7,384)
Other	(3,412)	(11,558)
Net assets acquired	34,546	61,874
Excess purchase price attributed to goodwill acquired	8,495	25,775
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 For Fiscal Year 2017
Fair value of consideration transferred:
Cash consideration		197,804
Working capital adjustment		120
Purchase price payable		2,278
Settlement of pre-existing relationship		(382)
Total consideration transferred		199,820
Fair value of assets acquired and liabilities assumed:
Cash		13,881
Accounts receivable		17,958
Inventories		25,581
Intangible assets		94,960
Fixed assets		14,487
Accounts payable		(3,151)
Other		(19,900)
Net assets acquired		143,816
Excess purchase price attributed to goodwill acquired		$ 56,004

Acquisitions - Fair Value of Am

Acquisitions - Fair Value of Amortizable Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2017
Vergokan
Business Acquisition [Line Items]
Intangible assets	$ 12,621
Other Acquisitions
Business Acquisition [Line Items]
Weighted Average Useful Life (Years)		9 years 10 months 24 days
Intangible assets	16,400	$ 40,100
Calpipe Industries
Business Acquisition [Line Items]
Weighted Average Useful Life (Years)		9 years 10 months 24 days
Intangible assets		$ 54,860
Customer relationships | Vergokan
Business Acquisition [Line Items]
Fair Value	$ 10,535
Weighted Average Useful Life (Years)	12 years
Customer relationships | Other Acquisitions
Business Acquisition [Line Items]
Fair Value	$ 15,400	$ 37,341
Weighted Average Useful Life (Years)	10 years	10 years
Customer relationships | Calpipe Industries
Business Acquisition [Line Items]
Fair Value		$ 50,680
Weighted Average Useful Life (Years)		10 years
Other | Vergokan
Business Acquisition [Line Items]
Fair Value	$ 2,086
Weighted Average Useful Life (Years)	9 years
Other | Other Acquisitions
Business Acquisition [Line Items]
Fair Value	$ 1,000	$ 2,759
Weighted Average Useful Life (Years)	9 years	8 years 1 month 6 days
Other | Calpipe Industries
Business Acquisition [Line Items]
Fair Value		$ 4,180
Weighted Average Useful Life (Years)		8 years 6 months

Acquisitions Acquisitions - Pro

Acquisitions Acquisitions - Pro Forma Results (Details) - Calpipe Industries $ in Thousands	12 Months Ended
	Sep. 30, 2017USD ($)
Business Acquisition [Line Items]
Pro forma net sales	$ 1,575,801
Pro forma net income	$ 91,362

Divestitures (Details)

Divestitures (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash consideration	$ 0	$ 42,631	$ 0
Goodwill divested during year		2,626
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 - Narra

Postretirement Benefits - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plans expense	$ 3,572	$ 3,600	$ 3,000
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	1,079	1,003
Estimated employer contributions in next fiscal year	$ 2,123
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%
Multi-employer Pension Plan
Multiemployer Plans [Line Items]
Liability	$ 5,658	$ 5,934

Postretirement Benefits - Net P

Postretirement Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Weighted-average assumptions used to determine net periodic pension cost during the period:
Discount rate	4.20%	3.70%	3.50%
Expected return on plan assets	6.30%	6.40%	7.00%
Pension Plan
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 0	$ 0	$ 2,049
Interest cost	4,665	4,098	3,793
Expected return on plan assets	(6,371)	(6,415)	(6,601)
Amortization of actuarial loss	101	343	1,303
Net periodic (benefit) cost	$ (1,605)	$ (1,974)	$ 544

Postretirement Benefits - Chang

Postretirement Benefits - Change in Benefit Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Change in benefit obligations:
Actuarial gain	$ 888
Amounts recognized in the consolidated balance sheets consist of:
Pension liabilities	(34,509)	$ (17,075)
Pension Plan
Change in benefit obligations:
Beginning balance, benefit obligations	120,957	128,852
Interest cost	4,665	4,098	$ 3,793
Actuarial gain	17,463	(7,130)
Benefits and administrative expenses paid	(5,125)	(4,863)
Ending balance, benefit obligations	137,960	120,957	128,852
Change in plan assets:
Beginning balance, fair value of plan assets	104,454	103,613
Actual return on plan assets	3,043	4,701
Employer contributions	1,079	1,003
Benefits and administrative expenses paid	(5,125)	(4,863)
Ending balance, fair value of plan assets	103,451	104,454	$ 103,613
Amounts recognized in the consolidated balance sheets consist of:
Pension assets	0	572
Pension liabilities	(34,509)	(17,075)
Net amount recognized	(34,509)	(16,503)
Amounts recognized in accumulated other comprehensive loss (before income taxes) consist of:
Net actuarial loss	(33,035)	(12,344)
Total loss recognized	$ (33,035)	$ (12,344)
Weighted-average assumptions used to determine pension benefit obligations at year end:
Discount rate	3.10%	4.20%

Postretirement Benefits - Benef

Postretirement Benefits - Benefit Obligation In Excess Of Plan Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan, Pension Plan with Project Benefit Obligation in Excess of Plan Assets [Abstract]
Accumulated benefit obligation	$ 137,959	$ 105,414
Fair value of plan assets	103,451	88,339
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets [Abstract]
Projected benefit obligation	137,959	105,414
Fair value of plan assets	$ 103,451	$ 88,339

Postretirement Benefits - Weigh

Postretirement Benefits - Weighted-Average Asset Allocations (Details) - Pension Plan	Sep. 30, 2019	Sep. 30, 2018
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%	59.00%
Debt securities
Defined Benefit Plan Disclosure [Line Items]
Plan asset allocations	40.00%	39.00%
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Plan asset allocations	2.00%	2.00%

Postretirement Benefits - Asset

Postretirement Benefits - Asset Allocations by Fair Value Hierarchy (Details) - Pension Plan - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 103,451	$ 104,454	$ 103,613
United States equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	38,974	38,801
Non-U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	20,725	22,763
Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	42,029	40,789
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,723	2,101
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	87,664	88,823
Level 1 | United States equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	32,545	32,447
Level 1 | Non-U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	18,182	20,164
Level 1 | Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	35,214	34,111
Level 1 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,723	2,101
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15,787	15,631
Level 2 | United States equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,429	6,354
Level 2 | Non-U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,543	2,599
Level 2 | Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,815	6,678
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, 2019USD ($)
Retirement Benefits [Abstract]
2020	$ 5,841
2021	6,159
2022	6,442
2023	6,670
2024	6,903
2025 to 2029	$ 37,071

Stock Incentive Plan - Narrativ

Stock Incentive Plan - Narrative (Details) - USD ($) $ in Thousands, shares in Millions	Jun. 10, 2016	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense		$ 11,798	$ 14,664	$ 12,788
Employee service share-based compensation, tax benefit from compensation expense		$ 2,949	3,681	$ 4,731
Omnibus Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance (shares)	1.6
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average period		1 year 7 months 20 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		1 year 8 months 12 days
Additional expense			$ 500

Stock Incentive Plan - Stock Op

Stock Incentive Plan - Stock Options Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	$ 7,374	$ 20,110	$ 12,168
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense	$ 2,575
Weighted-average period	1 year 6 months
Fair value of shares vested	$ 3,038	9,757	10,696
Tax benefit for tax deductions from stock options exercised	$ 4,031	$ 11,478	$ 9,530

Stock Incentive Plan - Stock _2

Stock Incentive Plan - Stock Options Assumptions (Details) - Stock Options	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	0.00%	0.00%	0.00%
Expected volatility	45.00%	40.00%	40.00%
Range of risk-free interest rates			1.95%
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 _3

Stock Incentive Plan - Stock Options Activity (Details) - Stock Options - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Shares (in thousands)
Outstanding, beginning balance (shares)	2,605	5,158	6,664
Granted (shares)	342	185	171
Exercised (shares)	(985)	(2,728)	(1,629)
Forfeited (shares)	(126)	(10)	(48)
Outstanding, ending balance (shares)	1,836	2,605	5,158
Vested (shares)	1,301
Weighted-Average Exercise Price
Outstanding, beginning balance (in dollars per share)	$ 9.91	$ 8.30	$ 7.76
Granted (in dollars per share)	23.45	20.01	21.45
Exercised (in dollars per share)	8.62	7.55	7.47
Forfeited (in dollars per share)	12.64	8.28	9.02
Outstanding, ending balance (in dollars per share)	12.94	9.91	8.30
Vested (in dollars per share)	9.46
Stock Options, Additional Disclosures
Weighted average grant date fair value of options granted	$ 11.35	$ 8.33	$ 8.84
Exercised, aggregate intrinsic value	$ 16,453	$ 42,512	$ 25,757
Outstanding, aggregate intrinsic value	31,957
Vested, aggregate intrinsic value	$ 27,185
Outstanding, weighted average remaining contractual term (in years)	5 years 8 months 19 days
Vested, weighted average remaining contractual term (in years)	4 years 7 months 17 days

Stock Incentive Plan - Restrict

Stock Incentive Plan - Restricted Stock Narrative (Details) - Restricted Stock Units (RSUs) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years
Unrecognized compensation expense	$ 6,460
Weighted-average period	1 year 8 months 12 days
Fair value of shares vested in period	$ 4,271	$ 1,969	$ 0

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, 2019	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Nonvested, beginning balance (shares)	650	435	0
Nonvested, granted (shares)	361	334	443
Nonvested, vested (shares)	(195)	(90)
Nonvested, forfeited (shares)	(142)	(29)	(8)
Nonvested, ending balance (shares)	674	650	435
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)	$ 19.91	$ 19.65	$ 0
Nonvested, granted (in dollars per share)	19.23	20.86	19.68
Nonvested, vested (in dollars per share)	21.51	21.95
Nonvested, forfeited (in dollars per share)	18.52	20.57	21.45
Nonvested, beginning of year (in dollars per share)	$ 19.37	$ 19.91	$ 19.65

Stock Incentive Plan - Performa

Stock Incentive Plan - Performance Share Units Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	Nov. 30, 2016	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Closing stock price (in dollars per share)		$ 17.90	$ 20.01	$ 21.45
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	3 years
Unrecognized compensation expense		$ 4,020
Weighted-average period		1 year 7 months 20 days
Nonvested, vested (shares)		0	0	0

Stock Incentive Plan - Perfor_2

Stock Incentive Plan - Performance Share Assumptions (Details) - Performance Shares - $ / shares	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	0.00%	0.00%	0.00%
Expected volatility, minimum	20.69%	17.21%	17.55%
Expected volatility, maximum	74.26%	79.15%	75.55%
Range of risk-free interest rates	2.84%		1.35%
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)	$ 18.84		$ 29.53
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 shares in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Nonvested, beginning balance (shares)	307	181	0
Shares issued (shares)	242	215	184
Nonvested, forfeited (shares)	(94)	(89)	(3)
Nonvested, ending balance (shares)	455	307	181
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)	$ 21.22	$ 23.87	$ 0
Grant date fair value (in dollars per share)	18.17	22.01	23.87
Nonvested, forfeited (in dollars per share)	20.55	28.53	23.87
Nonvested, beginning of year (in dollars per share)	$ 21.11	$ 21.22	$ 23.87

Restructuring Charges - Restruc

Restructuring Charges - Restructuring Reserves (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Restructuring Reserve [Roll Forward]
Beginning balance	$ 551	$ 737	$ 1,380
Charges	3,804	2,040	1,522
Utilization	(3,818)	(2,043)	(1,919)
Reversals		(191)	(266)
Exchange rate effects		8	20
Ending balance	537	551	737
Other/Corporate | Severance
Restructuring Reserve [Roll Forward]
Beginning balance	0	0	0
Charges	0	98	71
Utilization	0	(98)	(71)
Reversals		0	0
Exchange rate effects		0	0
Ending balance	0	0	0
Electrical Raceway | Operating Segments | Severance
Restructuring Reserve [Roll Forward]
Beginning balance	212	449	841
Charges	1,047	536	527
Utilization	(867)	(787)	(917)
Reversals		0	0
Exchange rate effects		14	(2)
Ending balance	392	212	449
Electrical Raceway | Operating Segments | Other
Restructuring Reserve [Roll Forward]
Beginning balance	310	0	0
Charges	2,544	1,130	439
Utilization	(2,854)	(820)	(209)
Reversals		0	(230)
Exchange rate effects		0	0
Ending balance	0	310	0
Mechanical Products & Solutions | Operating Segments | Severance
Restructuring Reserve [Roll Forward]
Beginning balance	0	278	0
Charges	213	97	422
Utilization	(68)	(178)	(166)
Reversals		(191)	0
Exchange rate effects		(6)	22
Ending balance	145	0	278
Mechanical Products & Solutions | Operating Segments | Other
Restructuring Reserve [Roll Forward]
Beginning balance	29	10	539
Charges	0	179	63
Utilization	(29)	(160)	(556)
Reversals		0	(36)
Exchange rate effects		0	0
Ending balance	$ 0	$ 29	$ 10

Restructuring Charges - Schedul

Restructuring Charges - Schedule of Restructuring Charges, Net (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Restructuring and Related Activities [Abstract]
Total restructuring charges, net	$ 3,804	$ 1,849	$ 1,256

Other Income, Net - Schedule of

Other Income, Net - Schedule of Other Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Other Income, Nonoperating [Abstract]
Gain on purchase of business	$ (7,384)	$ 0	$ 0
Gain on sale of a business	0	(27,575)	0
Gain on sale of joint venture	0	0	(5,774)
Undesignated foreign currency derivative instruments	(5,384)	(121)	2,741
Foreign exchange loss (gain) on intercompany loans	2,975	1,500	(2,038)
Debt modification costs	0	892	0
Pension-related benefits	(1,606)	(1,975)	(1,504)
Other	(79)	(69)	0
Other income, net	$ (11,478)	$ (27,348)	$ (6,575)

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, 2019	Sep. 30, 2018	Sep. 30, 2017
Components of income before income taxes:
United States	$ 170,780	$ 155,453	$ 116,580
Non-U.S	13,889	10,899	9,545
Income before income taxes	184,669	166,352	126,125
United States:
Federal	31,431	26,586	33,127
State	9,800	8,662	4,246
Non-U.S:	5,183	3,467	3,175
Current income tax expense	46,414	38,715	40,548
United States:
Federal	1,123	(7,149)	224
State	(340)	(1,793)	469
Non-U.S:	(1,579)	(66)	245
Deferred income (benefit) tax expense	(796)	(9,008)	938
Income tax expense	$ 45,618	$ 29,707	$ 41,486

Income Taxes - Differences Betw

Income Taxes - Differences Between Federal Income Tax Rate and Effective Income Tax Rate (Details)	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Statutory federal tax	21.00%	25.00%	35.00%
Adjustments to reconcile to the effective income tax rate:
State income taxes	4.00%	3.00%	3.00%
Change in tax rates	0.00%	(3.00%)	0.00%
Domestic manufacturing deduction	0.00%	(2.00%)	(2.00%)
Indemnified uncertain tax benefits	0.00%	(2.00%)	0.00%
Stock-based compensation	(1.00%)	(3.00%)	(3.00%)
Other	1.00%	0.00%	0.00%
Effective income tax rate	25.00%	18.00%	33.00%

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($) $ in Thousands	Dec. 22, 2017	Dec. 29, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Operating Loss Carryforwards [Line Items]
Effective income tax rate reconciliation, effect of Tax Cuts and Jobs Act of 2017, amount	$ 4,758	$ 4,758		$ 708
Tax benefit from release of indemnified uncertain tax positions			$ 7,566	$ 5,466	$ 3,589
Effective income tax rate reconciliation, Tax Cuts and Jobs Act			953
Effective income tax rate reconciliation, Tax Cuts and Jobs Act, foreign tax withholdings			635
Effective income tax rate reconciliation, acquisition of business			$ 1,551
Statutory federal tax			21.00%	25.00%	35.00%
Effective income tax reconciliation, excess tax benefit stock option			$ 1,537	$ 4,091
Release of indemnification tax positions				3,977
Tax benefit for manufacturing deduction				3,309	$ 2,761
State income tax expense				5,675	3,459
Operating loss carryforwards expiration term (year)			5 years
Cumulative loss position (year)			3 years
Unrecognized tax benefits			$ 794	1,364	3,578	$ 3,803
Accrued interest and penalties			54	78
Settlements			(814)	(3,107)	(288)
Undistributed income			52,404
Other Deferred Tax Assets
Operating Loss Carryforwards [Line Items]
Reestablished valuation allowance deferred tax assets			550
Tyco
Operating Loss Carryforwards [Line Items]
Settlements			(814)	(3,107)	(288)
Increase from settlments			244	893
Increase (decrease) in uncertain tax positions related to accrued penalties and interest			25	$ 3,139	$ 158
State
Operating Loss Carryforwards [Line Items]
Operating loss carrying forwards			43,403
Non-U.S.
Operating Loss Carryforwards [Line Items]
Operating loss carrying forwards			$ 40,457

Income Taxes - Components of Ne

Income Taxes - Components of Net Deferred Income Tax Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018
Deferred tax assets:
Accrued liabilities and reserves	$ 32,664	$ 32,099
Tax loss and credit carryforwards	13,296	12,435
Postretirement benefits	10,315	4,317
Inventory	8,813	11,220
Other	916	218
Deferred tax assets	66,004	60,289
Deferred tax liabilities:
Property, plant and equipment	(21,312)	(13,739)
Intangible assets	(47,887)	(48,375)
Loss on investment	(4,790)	(5,321)
Other	(3,660)	(1,775)
Deferred tax liabilities	(77,649)	(69,210)
Net deferred tax liability before valuation allowance	(11,645)	(8,921)
Valuation allowance	(7,764)	(7,427)
Net deferred tax liability	$ (19,409)	$ (16,348)

Income Taxes - Unrecognized Tax

Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance	$ 1,364	$ 3,578	$ 3,803
Additions based on tax positions related to prior years	244	893	63
Settlements	(814)	(3,107)	(288)
Ending balance	$ 794	$ 1,364	$ 3,578

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, 2019	Jun. 28, 2019	Mar. 29, 2019	Dec. 28, 2018	Sep. 30, 2018	Jun. 29, 2018	Mar. 30, 2018	Dec. 29, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net income	$ 45,997	$ 36,550	$ 29,555	$ 26,949	$ 32,699	$ 34,199	$ 42,558	$ 27,189	$ 139,051	$ 136,645	$ 84,639
Less: Undistributed earnings allocated to participating securities									3,726	2,456	241
Net income available to common shareholders									$ 135,325	$ 134,189	$ 84,398
Denominator:
Basic weighted average common shares outstanding (shares)									46,577	51,791	63,420
Effect of dilutive securities: non-participating employee stock options (shares)									1,200	2,298	3,134
Diluted weighted average common shares outstanding (shares)									47,777	54,089	66,554
Basic (in dollars per share)	$ 0.96	$ 0.77	$ 0.62	$ 0.56	$ 0.69	$ 0.73	$ 0.83	$ 0.43	$ 2.91	$ 2.59	$ 1.33
Diluted (in dollars per share)	$ 0.94	$ 0.75	$ 0.61	$ 0.54	$ 0.66	$ 0.70	$ 0.79	$ 0.41	$ 2.83	$ 2.48	$ 1.27
Common Stock
Denominator:
Antidilutive securities excluded from computation of earnings per share, amount									400	300	200

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, 2019	Sep. 30, 2018	Sep. 30, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before reclassifications, net of tax		$ (23,003)	$ 1,215
Amounts reclassified from accumulated other comprehensive loss		76	329
Reclassification of stranded tax benefits	[1]	0
Defined benefit pension items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period		(6,048)	(10,445)
Other comprehensive income (loss), before reclassifications, net of tax		(15,513)	4,068
Amounts reclassified from accumulated other comprehensive loss		76	329	$ 806
Other comprehensive income (loss), net of tax		(15,437)	4,397
Reclassification of stranded tax benefits	[1]	(2,333)
Balance at end of period		(23,818)	(6,048)	(10,445)
Accumulated Foreign Currency Adjustment Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period		(10,390)	(7,537)
Other comprehensive income (loss), before reclassifications, net of tax		(7,490)	(2,853)
Amounts reclassified from accumulated other comprehensive loss		0	0
Other comprehensive income (loss), net of tax		(7,490)	(2,853)
Reclassification of stranded tax benefits	[1]	0
Balance at end of period		(17,880)	(10,390)	(7,537)
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period		(16,438)	(17,982)
Other comprehensive income (loss), net of tax		(22,927)	1,544
Reclassification of stranded tax benefits	[1]	(2,333)
Balance at end of period		$ (41,698)	$ (16,438)	$ (17,982)
[1]	(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, 2019	Sep. 30, 2018	Sep. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from accumulated other comprehensive loss	$ 76	$ 329
Net actuarial loss for pension benefit plans	888
Tax expense
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of net loss (included within other income, net)	101	343	$ 1,303
Tax expense	(25)	(14)	(497)
Amounts reclassified from accumulated other comprehensive loss	$ 76	$ 329	$ 806

Inventories, Net - Schedule of

Inventories, Net - Schedule of Company Inventories (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018
Inventory Disclosure [Abstract]
Purchased materials and manufactured parts, net	$ 52,742	$ 58,572
Work in process, net	21,424	21,769
Finished goods, net	151,924	141,412
Inventories, net	$ 226,090	$ 221,753

Inventories, Net - Narrative (D

Inventories, Net - Narrative (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018
Inventory Disclosure [Abstract]
Inventories at lower of LIFO cost or market (%)	72.00%	80.00%
Inventory, LIFO reserve	$ 3,138	$ 26,340
Excess and obsolete inventory reserve	$ 14,295	$ 12,909

Property, Plant and Equipment -

Property, Plant and Equipment - Schedule of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018
Property, Plant and Equipment [Line Items]
Property, plant and equipment	$ 520,533	$ 438,792
Accumulated depreciation	(259,830)	(225,684)
Property, plant and equipment, net	260,703	213,108
Land
Property, Plant and Equipment [Line Items]
Property, plant and equipment	19,897	13,295
Buildings and related improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment	127,061	108,758
Machinery and equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment	318,421	262,078
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment	9,055	7,382
Software
Property, Plant and Equipment [Line Items]
Property, plant and equipment	24,835	30,502
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment	$ 21,264	$ 16,777

Property, Plant and Equipment_2

Property, Plant and Equipment - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 39,471	$ 34,786	$ 32,320

Goodwill and Intangible Asset_2

Goodwill and Intangible Assets - Goodwill (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Goodwill [Roll Forward]
Beginning balance	$ 170,129	$ 147,716
Goodwill divested during year		(2,626)
Goodwill acquired during year	18,124	813
Purchase price adjustments		24,527
Exchange rate effects	(2,022)	(301)
Ending balance	186,231	170,129
Electrical Raceway
Goodwill [Roll Forward]
Beginning balance	133,566	108,527
Goodwill divested during year		0
Goodwill acquired during year	18,124	813
Purchase price adjustments		24,527
Exchange rate effects	(2,022)	(301)
Ending balance	149,668	133,566
Mechanical Products & Solutions
Goodwill [Roll Forward]
Beginning balance	36,563	39,189
Goodwill divested during year		(2,626)
Goodwill acquired during year	0	0
Purchase price adjustments		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, 2019	Sep. 30, 2018	Sep. 30, 2017
Indefinite-lived Intangible Assets [Line Items]
Amortization expense	$ 32,876	$ 32,104	$ 22,407
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, 2019	Sep. 30, 2018
Amortizable intangible assets:
Gross Carrying Value	$ 372,342	$ 346,298
Accumulated Amortization	(179,538)	(147,262)
Net Carrying Value	192,804	199,036
Intangible Assets, Net (Excluding Goodwill) [Abstract]
Gross Carrying Value	465,222	439,178
Net Carrying Value	285,684	291,916
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	353,256	330,295
Accumulated Amortization	(171,777)	(141,401)
Net Carrying Value	$ 181,479	188,894
Customer relationships | Weighted Average
Amortizable intangible assets:
Weighted Average Useful Life (Years)	11 years
Other
Amortizable intangible assets:
Gross Carrying Value	$ 19,086	16,003
Accumulated Amortization	(7,761)	(5,861)
Net Carrying Value	$ 11,325	$ 10,142
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, 2019USD ($)
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
2020	$ 32,400
2021	32,320
2022	30,893
2023	30,575
2024	26,101
2025 and thereafter	$ 40,515

Debt - Schedule of Debt (Detail

Debt - Schedule of Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018
Debt Instrument [Line Items]
Deferred financing costs	$ (6,569)	$ (8,194)
Total debt	845,317	904,247
Less: Current portion	0	26,561
Long-term debt	845,317	877,686
Secured Debt | First Lien Term Loan Facility due December 22, 2023
Debt Instrument [Line Items]
Carrying value	851,361	912,162
Other	$ 525	$ 279

Debt - Contractual Obligation (

Debt - Contractual Obligation (Details) $ in Thousands	Sep. 30, 2019USD ($)
Debt Disclosure [Abstract]
2020	$ 0
2021	0
2022	0
2023	0
2024	852,120
2025 and thereafter	$ 0

Debt - First Lien Loan - Narrat

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

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, 2019	Sep. 30, 2018	Sep. 30, 2017
Debt Instrument [Line Items]
Loss (gain) on extinguishment of debt				$ 0	$ 0	$ (9,805,000)
Repayments of secured debt				40,000,000	1,217,000	641,100,000
Payments for repurchase of common stock				24,419,000	411,775,000	13,938,000
Debt modification costs				0	892,000	$ 0
Secured Debt | First Lien Term Loan Facility due December 22, 2023
Debt Instrument [Line Items]
Carrying value				$ 851,361,000	$ 912,162,000
Secured Debt | New First Lien Term Loan Facility
Debt Instrument [Line Items]
Carrying value	$ 425,000,000
Repurchase of common stock	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 [Member]
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%
LIBOR floor (%)			1.00%
Atkore International | Secured Debt | Second Lien Term Loan Facility due October 9, 2021
Debt Instrument [Line Items]
Credit agreement			$ 250,000,000
Redemption price (%)			99.00%
Atkore International | Secured Debt | Second Lien Term Loan Facility due October 9, 2021 | LIBOR
Debt Instrument [Line Items]
Margin (%)			6.75%
LIBOR floor (%)			1.00%
Atkore International | Secured Debt | New First Lien Term Loan Facility
Debt Instrument [Line Items]
Credit agreement		$ 500,000
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) - Atkore International - USD ($)	Dec. 22, 2016	Dec. 21, 2016	Dec. 22, 2016	Sep. 30, 2019	Sep. 30, 2018
Domestic Line of Credit | ABL Credit Facility | LIBOR | Minimum
Debt Instrument [Line Items]
Applicable margin (%)	1.25%
Domestic Line of Credit | ABL Credit Facility | LIBOR | Maximum
Debt Instrument [Line Items]
Applicable margin (%)	1.75%
Domestic Line of Credit | ABL Credit Facility | Alternate Base Rate | Minimum
Debt Instrument [Line Items]
Applicable margin (%)	0.25%
Domestic Line of Credit | ABL Credit Facility | Alternate Base Rate | Maximum
Debt Instrument [Line Items]
Applicable margin (%)	0.75%
Foreign Line of Credit | ABL Credit Facility
Debt Instrument [Line Items]
Line of credit facility, unused capacity, commitment fee percentage (%)		0.25%		0.375%
Foreign Line of Credit | ABL Credit Facility | Bankers Acceptance Rate | Minimum
Debt Instrument [Line Items]
Applicable margin (%)	1.25%
Foreign Line of Credit | ABL Credit Facility | Bankers Acceptance Rate | Maximum
Debt Instrument [Line Items]
Applicable margin (%)	1.75%
Foreign Line of Credit | ABL Credit Facility | Prime Rate | Minimum
Debt Instrument [Line Items]
Applicable margin (%)	0.25%
Foreign Line of Credit | ABL Credit Facility | Prime Rate | Maximum
Debt Instrument [Line Items]
Applicable margin (%)	0.75%
Line of Credit | ABL Credit Facility
Debt Instrument [Line Items]
Line of credit facility, unused capacity, commitment fee percentage (%)			0.50%
Fair value of amount outstanding				$ 0	$ 0
Aggregate commitments				325,000,000
Credit availability				$ 301,882,000	$ 315,119,000
Borrowing base percentage of eligible accounts receivable (%)				85.00%
Borrowing base percentage of eligible inventory (plus) (%)				80.00%
Debt instrument, covenant, borrowing base, percentage of inventory subject to certain limitations (%)				85.00%
Secured Debt | New First Lien Term Loan Facility | LIBOR
Debt Instrument [Line Items]
Applicable margin (%)	3.00%
Secured Debt | New First Lien Term Loan Facility | Alternate Base Rate
Debt Instrument [Line Items]
Applicable margin (%)	2.00%

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Details) $ in Thousands	12 Months Ended
	Sep. 30, 2019USD ($)	Sep. 30, 2019GBP (£)	Sep. 30, 2018USD ($)	Sep. 30, 2018GBP (£)	Sep. 30, 2017GBP (£)
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Forward currency contracts | £		£ 45,000,000		£ 49,100,000	£ 0
Level 1 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Forward currency contracts	$ 0		$ 0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets held for sale			650
Level 3 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Forward currency contracts	$ 0		$ 0
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	5 years

Fair Value Measurements - Asset

Fair Value Measurements - Assets and Liabilities Measured at Fair Value (Details) - Recurring $ in Thousands	Sep. 30, 2019USD ($)	Sep. 30, 2019GBP (£)	Sep. 30, 2018USD ($)	Sep. 30, 2018GBP (£)	Sep. 30, 2017GBP (£)
Assets
Forward currency contracts | £		£ 45,000,000		£ 49,100,000	£ 0
Level 1
Assets
Cash equivalents	$ 72,132		$ 28,175
Forward currency contracts	0		0
Liabilities
Forward currency contracts	0		0
Level 2
Assets
Cash equivalents	0		0
Forward currency contracts	3,420		0
Liabilities
Forward currency contracts	0		1,857
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, 2019	Sep. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value	$ 852,120	$ 913,100
Fair value	853,543	916,113
First Lien Term Loan Facility due December 22, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value	852,120	913,100
Fair value	$ 853,543	$ 916,113

Commitments and Contingencies -

Commitments and Contingencies - Narrative (Details) - USD ($)	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Aug. 03, 2018	Mar. 31, 2017
Loss Contingencies [Line Items]
Purchase obligation for fiscal 2020	$ 97,011,000
Purchase obligation for fiscal 2021	1,826,000
Purchase obligation thereafter	411,000
Total rental expense on all operating leases	17,077,000	$ 17,403,000	$ 12,941,000
Total settlement for proir cap agreement deductible	13,000,000
Special Products Claims and Other Product Liabilities
Loss Contingencies [Line Items]
Product liability	2,424,000	6,755,000
Special Products Claims and Other Product Liabilities | Minimum
Loss Contingencies [Line Items]
Probable losses	1,000,000
Special Products Claims and Other Product Liabilities | Maximum
Loss Contingencies [Line Items]
Probable losses	8,000,000
Special Products Claims
Loss Contingencies [Line Items]
Total settlement for cap agreement deductible	$ 12,000,000
Antidumping Duty, Malleable Iron Pipe Fittings from China
Loss Contingencies [Line Items]
Product liability					$ 7,501,000
Restatement Adjustment | Antidumping Duty, Malleable Iron Pipe Fittings from China
Loss Contingencies [Line Items]
Product liability		$ (7,171,000)		$ (7,171,000)

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Minimum Future Operating Lease Payments (Details) $ in Thousands	Sep. 30, 2019USD ($)
Commitments and Contingencies Disclosure [Abstract]
2020	$ 13,526
2021	11,592
2022	8,666
2023	6,362
2024	5,097
2025 and thereafter	6,938
Total	$ 52,181

Guarantees - Narrative (Details

Guarantees - Narrative (Details) $ in Thousands	Sep. 30, 2019USD ($)
Workers' compensation and general liability insurance policies
Guarantor Obligations [Line Items]
Guarantees	$ 10,501
Surety bond
Guarantor Obligations [Line Items]
Guarantees	$ 18,200

Related Party Transactions - Na

Related Party Transactions - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands, shares in Millions	Jan. 22, 2018	Jun. 15, 2016	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Related Party Transaction [Line Items]
Common stock, par value (in dollars per share)			$ 0.01	$ 0.01
Repurchase of common stock			$ 24,419	$ 411,775	$ 13,938
Related party net sales				$ 6,000	$ 8,300
Consulting and Termination Fees
Related Party Transaction [Line Items]
Related party fees						$ 15,425
Termination Fee
Related Party Transaction [Line Items]
Related party fees		$ 12,800
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]
Stock repurchase program, number of shares authorized to be repurchased (shares)	17.2

Segment Information - Narrative

Segment Information - Narrative (Details)	12 Months Ended
	Sep. 30, 2019segmentemployee	Sep. 30, 2018
Concentration Risk [Line Items]
Number of operating segments | segment	2
Number of reportable segments | segment	2
Customer percentage of sales or accounts receivable (%)	10.00%
Employees represented by a union under a collective bargaining agreement (approximately) (%)	27.00%
Number of employees affected by expiring of contracts | employee	15
Bargaining unit of employees | employee	400
Sonepar Management US, Inc.
Concentration Risk [Line Items]
Concentration risk (%)		11.00%

Segment Information - Shared As

Segment Information - Shared Assets (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 28, 2019	Mar. 29, 2019	Dec. 28, 2018	Sep. 30, 2018	Jun. 29, 2018	Mar. 30, 2018	Dec. 29, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Net sales	$ 501,710	$ 493,491	$ 469,309	$ 452,028	$ 477,567	$ 498,014	$ 445,000	$ 414,558	$ 1,916,538	$ 1,835,139	$ 1,503,934
Capital Expenditures									34,860	38,501	25,122
Total Assets	1,436,995				1,324,060				1,436,995	1,324,060	1,215,092
Electrical Raceway
Segment Reporting Information [Line Items]
Net sales									1,442,278	1,365,067	1,093,500
MP&S
Segment Reporting Information [Line Items]
Net sales									474,260	470,072	410,434
Intersegment Sales
Segment Reporting Information [Line Items]
Net sales									(1,215)	(1,625)	(1,381)
Intersegment Sales | Electrical Raceway
Segment Reporting Information [Line Items]
Net sales									1,215	1,544	1,283
Intersegment Sales | MP&S
Segment Reporting Information [Line Items]
Net sales									0	81	98
Operating Segments
Segment Reporting Information [Line Items]
Adjusted EBITDA									362,625	306,599	253,038
Operating Segments | Electrical Raceway
Segment Reporting Information [Line Items]
Adjusted EBITDA									292,585	255,260	189,351
Capital Expenditures									19,856	16,389	13,037
Total Assets	871,771				751,024				871,771	751,024	757,775
Operating Segments | MP&S
Segment Reporting Information [Line Items]
Adjusted EBITDA									70,040	51,339	63,687
Capital Expenditures									13,934	14,267	8,212
Total Assets	252,708				291,164				252,708	291,164	306,229
Unallocated
Segment Reporting Information [Line Items]
Capital Expenditures									1,070	7,845	3,873
Total Assets	$ 312,516				$ 281,872				$ 312,516	$ 281,872	$ 151,088

Segment Information - Reconcili

Segment Information - Reconciliation of Operating Segment Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Unallocated expenses	$ (38,217)	$ (35,050)	$ (25,430)
Depreciation and amortization	(72,347)	(66,890)	(54,727)
Interest expense, net	(50,473)	(40,694)	(26,598)
Gain (loss) on extinguishment of debt	0	0	(9,805)
Restructuring charges	(3,804)	(1,849)	(1,256)
Stock-based compensation	(11,798)	(14,664)	(12,788)
Certain legal matters	0	4,833	(7,551)
Transaction costs	(1,200)	(9,314)	(4,779)
Gain on purchase of business	7,384	0	0
Gain on sale of a business	0	27,575	0
Gain on sale of joint venture	0	0	5,774
Other	(7,501)	(4,194)	10,247
Income before income taxes	184,669	166,352	126,125
Operating Segments
Segment Reporting Information [Line Items]
Operating segment Adjusted EBITDA	362,625	306,599	253,038
Operating Segments | Electrical Raceway
Segment Reporting Information [Line Items]
Operating segment Adjusted EBITDA	292,585	255,260	189,351
Operating Segments | MP&S
Segment Reporting Information [Line Items]
Operating segment Adjusted EBITDA	$ 70,040	$ 51,339	$ 63,687

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, 2019	Jun. 28, 2019	Mar. 29, 2019	Dec. 28, 2018	Sep. 30, 2018	Jun. 29, 2018	Mar. 30, 2018	Dec. 29, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	$ 264,966				$ 214,715				$ 264,966	$ 214,715	$ 215,671
Net sales	501,710	$ 493,491	$ 469,309	$ 452,028	477,567	$ 498,014	$ 445,000	$ 414,558	1,916,538	1,835,139	1,503,934
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	219,614				201,101				219,614	201,101	202,823
Net sales									1,689,194	1,651,636	1,367,907
Other Americas
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	147				138				147	138	164
Net sales									33,485	43,013	37,908
Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	43,207				11,090				43,207	11,090	9,306
Net sales									142,279	90,915	55,181
Asia-Pacific
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	$ 1,998				$ 2,386				1,998	2,386	3,378
Net sales									$ 51,580	$ 49,575	$ 42,938

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, 2019	Jun. 28, 2019	Mar. 29, 2019	Dec. 28, 2018	Sep. 30, 2018	Jun. 29, 2018	Mar. 30, 2018	Dec. 29, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Revenue from External Customer [Line Items]
Net sales	$ 501,710	$ 493,491	$ 469,309	$ 452,028	$ 477,567	$ 498,014	$ 445,000	$ 414,558	$ 1,916,538	$ 1,835,139	$ 1,503,934
Electrical Raceway
Revenue from External Customer [Line Items]
Net sales									1,442,278	1,365,067	1,093,500
Electrical Raceway | Metal Electrical Conduit and Fittings
Revenue from External Customer [Line Items]
Net sales									546,533	517,935	349,239
Electrical Raceway | Armored Cable and Fittings
Revenue from External Customer [Line Items]
Net sales									360,494	336,388	323,070
Electrical Raceway | PVC Electrical Conduit & Fittings
Revenue from External Customer [Line Items]
Net sales									292,243	311,811	265,389
Electrical Raceway | Other raceway products
Revenue from External Customer [Line Items]
Net sales									243,008	198,933	155,802
Mechanical Products & Solutions
Revenue from External Customer [Line Items]
Net sales									474,260	470,072	410,434
Mechanical Products & Solutions | Mechanical Pipe
Revenue from External Customer [Line Items]
Net sales									259,613	253,381	211,245
Mechanical Products & Solutions | Other MP&S products
Revenue from External Customer [Line Items]
Net sales									$ 214,647	$ 216,691	$ 199,189

Quarterly Operating Results (_2

Quarterly Operating Results (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	Aug. 21, 2019	Dec. 22, 2017	Sep. 30, 2019	Jun. 28, 2019	Mar. 29, 2019	Dec. 28, 2018	Sep. 30, 2018	Jun. 29, 2018	Mar. 30, 2018	Dec. 29, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Aug. 03, 2018	Mar. 31, 2017
Debt Instrument [Line Items]
Net sales			$ 501,710	$ 493,491	$ 469,309	$ 452,028	$ 477,567	$ 498,014	$ 445,000	$ 414,558	$ 1,916,538	$ 1,835,139	$ 1,503,934
Gross profit			143,722	126,134	117,088	110,256	111,731	120,329	109,157	96,867	497,200	438,084	361,270
Net income			$ 45,997	$ 36,550	$ 29,555	$ 26,949	$ 32,699	$ 34,199	$ 42,558	$ 27,189	$ 139,051	$ 136,645	$ 84,639
Net income per share
Basic (in dollars per share)			$ 0.96	$ 0.77	$ 0.62	$ 0.56	$ 0.69	$ 0.73	$ 0.83	$ 0.43	$ 2.91	$ 2.59	$ 1.33
Diluted (in dollars per share)			$ 0.94	$ 0.75	$ 0.61	$ 0.54	$ 0.66	$ 0.70	$ 0.79	$ 0.41	$ 2.83	$ 2.48	$ 1.27
Gain (loss) on extinguishment of debt											$ 0	$ 0	$ (9,805)
Effective income tax rate reconciliation, effect of Tax Cuts and Jobs Act of 2017, amount		$ 4,758								$ 4,758		708
Bargain purchase gain											$ 7,384	0	$ 0
Antidumping Duty, Malleable Iron Pipe Fittings from China
Net income per share
Product liability															$ 7,501
FlexHead Industries, Inc and SprinkFLEX, LLC | Disposed of by Sale
Net income per share
Gain on sale of business									$ 26,737
Gain on sale of business, working capital adjustment								$ 838
Cor-Tek
Net income per share
Bargain purchase gain	$ 7,384		$ 7,384
Restatement Adjustment | Antidumping Duty, Malleable Iron Pipe Fittings from China
Net income per share
Product liability							$ (7,171)					$ (7,171)		$ (7,171)

Schedule I - Condensed Financ_2

Schedule I - Condensed Financial Information - Balance Sheets (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Assets
Total Assets	$ 1,436,995	$ 1,324,060	$ 1,215,092
Liabilities and Equity
Total Liabilities	1,204,059	1,202,001
Equity:
Common stock, $0.01 par value, 1,000,000,000 shares authorized, 46,955,163 and 47,079,645 shares issued and outstanding, respectively	471	472
Treasury stock, held at cost, 260,900 and 260,900 shares, respectively	(2,580)	(2,580)
Additional paid-in capital	477,139	457,978
Accumulated deficit	(200,396)	(317,373)
Accumulated other comprehensive loss	(41,698)	(16,438)
Total Equity	232,936	122,059	$ 360,871	$ 257,246
Total Liabilities and Equity	1,436,995	1,324,060
Atkore International Group Inc.
Assets
Investment in subsidiary	232,936	122,059
Total Assets	232,936	122,059
Liabilities and Equity
Total Liabilities	0	0
Equity:
Additional paid-in capital	477,139	457,978
Total Equity	232,936	122,059
Total Liabilities and Equity	$ 232,936	$ 122,059

Schedule I - Condensed Financ_3

Schedule I - Condensed Financial Information - Balance Sheets (Parenthetical) (Details) - $ / shares	Sep. 30, 2019	Sep. 30, 2018
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)	46,955,163	47,079,645
Common stock, shares outstanding (shares)	46,955,163	47,079,645
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)	46,955,163	47,079,645
Common stock, shares outstanding (shares)	46,955,163	47,079,645
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, 2019	Jun. 28, 2019	Mar. 29, 2019	Dec. 28, 2018	Sep. 30, 2018	Jun. 29, 2018	Mar. 30, 2018	Dec. 29, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Condensed Income Statements, Captions [Line Items]
Net income	$ 45,997	$ 36,550	$ 29,555	$ 26,949	$ 32,699	$ 34,199	$ 42,558	$ 27,189	$ 139,051	$ 136,645	$ 84,639
Other comprehensive (loss) income of subsidiary, net of tax									(22,927)	1,544	7,968
Comprehensive income									116,124	138,189	92,607
Atkore International Group Inc.
Condensed Income Statements, Captions [Line Items]
Net income									139,051	136,645	84,639
Other comprehensive (loss) income of subsidiary, net of tax									(22,927)
Comprehensive income									$ 116,124	$ 138,189	$ 92,607

Schedule I - Condensed Financ_5

Schedule I - Condensed Financial Information - Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Cash Flows from Operating Activities:
Net cash provided by operating activities	$ 209,694	$ 145,703	$ 121,654
Cash Flows from Investing Activities:
Net cash (used for) provided by investing activities	(133,101)	2,514	(205,833)
Cash Flows from Financing Activities:
Issuance of common stock	7,374	20,110	12,168
Repurchase of common stock	(24,419)	(411,775)	(13,938)
Net cash used for financing activities	(78,180)	(65,931)	(67,760)
Increase (decrease) in cash and cash equivalents	(3,247)	80,944	(154,561)
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	24,419	411,775	13,938
Distribution paid to subsidiary	(7,374)	(20,110)	(12,168)
Net cash (used for) provided by investing activities	17,045	391,665	1,770
Cash Flows from Financing Activities:
Issuance of common stock		20,110	12,168
Repurchase of common stock	(24,419)	(411,775)	(13,938)
Net cash used for financing activities	(17,045)	(391,665)	(1,770)
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, 2019	Sep. 30, 2018	Sep. 30, 2017	Dec. 22, 2010
Condensed Financial Statements, Captions [Line Items]
Payment to redeem shares	$ 250,000	$ 250,000
Atkore International Group Inc.
Condensed Financial Statements, Captions [Line Items]
Distribution received from subsidiary			$ 24,419	$ 411,775	$ 13,938
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	$ 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, 2019	Sep. 30, 2018	Sep. 30, 2017
Accounts Receivable Allowance for Doubtful Accounts
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	$ (1,762)	$ (1,239)	$ (1,006)
Additional (Charges)/Benefit to Income	(970)	(389)	(243)
Write offs and Other	124	(134)	10
Balance at End of Year	(2,608)	(1,762)	(1,239)
Deferred Tax Valuation Allowance
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	(7,427)	(9,512)	(8,658)
Additional (Charges)/Benefit to Income	(346)	428	(1,164)
Write offs and Other	9	1,657	310
Balance at End of Year	$ (7,764)	$ (7,427)	$ (9,512)