Atkore (ATKR) 10-K 27 Nov 18 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Nov. 18, 2018	Mar. 30, 2018
Document and Entity Information [Abstract]
Entity Registrant Name	Atkore International Group Inc.
Entity Central Index Key	1,666,138
Document Type	10-K
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2,018
Amendment Flag	false
Document Fiscal Period Focus	FY
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Document Fiscal Year Focus	--09-30
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		47,079,645
Entity Public Float			$ 772.1

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Net sales	$ 1,835,139	$ 1,503,934	$ 1,523,384
Cost of sales	1,397,055	1,142,664	1,155,808
Gross profit	438,084	361,270	367,576
Selling, general and administrative	226,282	182,910	219,743
Intangible asset amortization	32,104	22,407	22,238
Asset impairment charges	0	0	129
Operating income	179,698	155,953	125,466
Interest expense, net	40,694	26,598	41,798
Loss (gain) on extinguishment of debt	0	9,805	(1,661)
Other income, net	(27,348)	(6,575)	(1,452)
Income before income taxes	166,352	126,125	86,781
Income tax expense	29,707	41,486	27,985
Net income	$ 136,645	$ 84,639	$ 58,796
Net income per share
Basic (in dollars per share)	$ 2.59	$ 1.33	$ 0.94
Diluted (in dollars per share)	$ 2.48	$ 1.27	$ 0.94

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 136,645	$ 84,639	$ 58,796
Other comprehensive (loss), net of tax:
Change in foreign currency translation adjustment	(2,853)	1,221	(858)
Change in unrecognized loss related to pension benefit plans, net of tax expense (benefit) of ($1,362), ($3,356), $2,680, respectively	4,397	6,747	(4,059)
Total other comprehensive income (loss)	1,544	7,968	(4,917)
Comprehensive income	$ 138,189	$ 92,607	$ 53,879

Consolidated Statements of Co_2

Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Change in unrecognized loss related to pension benefit plans, tax benefit	$ (1,362)	$ (3,356)	$ 2,680

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Current Assets:
Cash and cash equivalents	$ 126,662	$ 45,718
Accounts receivable, less allowance for doubtful accounts of $1,762 and $1,239, respectively	265,147	224,427
Inventories, net	221,753	200,003
Prepaid expenses and other current assets	33,576	35,611
Total current assets	647,138	505,759
Property, plant and equipment, net	213,108	208,619
Intangible assets, net	291,916	344,289
Goodwill	170,129	147,716
Deferred income taxes	162	1,657
Non-trade receivables	1,607	7,052
Total Assets	1,324,060	1,215,092
Current Liabilities:
Short-term debt and current maturities of long-term debt	26,561	4,215
Accounts payable	156,525	125,618
Income tax payable	542	2,581
Accrued compensation and employee benefits	33,350	26,387
Other current liabilities	55,769	53,036
Total current liabilities	272,747	211,837
Long-term debt	877,686	571,863
Deferred income taxes	16,510	17,464
Other long-term tax liabilities	1,443	6,771
Pension liabilities	17,075	25,239
Other long-term liabilities	16,540	21,047
Total Liabilities	1,202,001	854,221
Equity:
Common stock, $0.01 par value, 1,000,000,000 shares authorized, 47,079,645 and 63,305,434 shares issued and outstanding, respectively	472	634
Treasury stock, held at cost, 260,900 and 260,900 shares, respectively	(2,580)	(2,580)
Additional paid-in capital	457,978	423,232
Accumulated deficit	(317,373)	(42,433)
Accumulated other comprehensive loss	(16,438)	(17,982)
Total Equity	122,059	360,871
Total Liabilities and Equity	$ 1,324,060	$ 1,215,092

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Statement of Financial Position [Abstract]
Accounts receivable, less allowance for doubtful accounts of $1,762 and $1,239, respectively	$ 1,762	$ 1,239
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (shares)	1,000,000,000	1,000,000,000
Common stock, shares issued (shares)	47,079,645	63,305,434
Common stock, shares outstanding (shares)	47,079,645	63,305,434
Treasury stock (shares)	260,900	260,900

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Operating activities:
Net income	$ 136,645	$ 84,639	$ 58,796
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on sale of a business	(27,575)	(5,093)	(357)
Impairment of assets	0	0	129
Depreciation and amortization	66,890	54,727	55,017
Amortization of debt issuance costs and original issue discount	1,542	1,446	3,586
Deferred income taxes	(9,008)	938	2,556
Loss (gain) on extinguishment of debt	0	9,805	(1,661)
Provision for losses on accounts receivable and inventory	7,241	1,333	3,021
Stock-based compensation expense	14,664	12,788	21,127
Other adjustments to net income	2,023	896	(190)
Changes in operating assets and liabilities, net of effects from acquisitions
Accounts receivable	(44,419)	(13,445)	24,538
Inventories	(36,528)	(10,301)	(2,437)
Prepaid expenses and other current assets	(1,762)	(3,074)	(2,986)
Accounts payable	31,667	8,673	4,061
Income taxes	(3,179)	(9,138)	1,005
Accrued and other liabilities	7,243	(11,232)	(9,551)
Other, net	259	(1,308)	(8)
Net cash provided by operating activities	145,703	121,654	156,646
Investing activities:
Capital expenditures	(38,501)	(25,122)	(16,830)
Proceeds from sale of properties, plant and equipment	349	100	75
Proceeds from divestiture of business	42,631	0	0
Proceeds from sale of assets held for sale	0	3,024	2,400
Acquisitions of businesses, net of cash acquired	(3,467)	(183,923)	0
Proceeds from sale of an investment	0	0	1,328
Other, net	1,502	88	132
Net cash provided by (used for) investing activities	2,514	(205,833)	(12,895)
Financing activities:
Borrowings under credit facility	358,000	97,000	0
Repayments under credit facility	(443,000)	(12,000)	0
Repayments of short-term debt	(8,150)	(4,200)	(1,619)
Issuance of long-term debt	426,217	498,750	0
Repayments of long-term debt	(1,217)	(641,100)	(22,175)
Issuance of common stock	20,110	12,168	52
Repurchase of common stock	(411,775)	(13,938)	0
Payments for debt financing costs and fees	(5,955)	(4,375)	0
Other, net	(161)	(65)	(166)
Net cash (used for) financing activities	(65,931)	(67,760)	(23,908)
Effects of foreign exchange rate changes on cash and cash equivalents	(1,342)	(2,622)	(162)
Increase (decrease) in cash and cash equivalents	80,944	(154,561)	119,681
Cash and cash equivalents at beginning of period	45,718	200,279	80,598
Cash and cash equivalents at end of period	126,662	45,718	200,279
Supplementary Cash Flow information
Interest paid	39,898	26,131	49,855
Income taxes paid, net of refunds	41,601	49,813	30,859
Capital expenditures, not yet paid	916	1,330	525
Reclassification of stock-based compensation liability	$ 0	$ 0	$ 43,870

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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect adjustment for a change in accounting principle	$ 1,303
Beginning balance (shares) at Sep. 25, 2015		62,453
Beginning balance at Sep. 25, 2015	156,277	$ 626	$ (2,580)	$ 352,505	$ (173,241)	$ (21,033)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	58,796				58,796
Other comprehensive income	(4,917)					(4,917)
Modification of liability award to equity based compensation	43,870
Stock-based compensation (shares)		0
Stock-based compensation		$ 0
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition	1,865			1,865
Issuance of common stock (shares)		5
Stock-based compensation	52			52
Ending balance (shares) at Sep. 30, 2016		62,458
Ending 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	7,968					7,968
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition	12,788			12,788
Issuance of common stock (shares)		1,628
Stock-based compensation	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	$ 0	0		136,645	0
Other comprehensive income	1,544			0	0	1,544
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition	14,664			14,664
Issuance of common stock (shares)		2,795
Stock-based compensation	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)

Basis of Presentation and Summa

Basis of Presentation and Summary of Significant Accounting Policies	12 Months Ended
	Sep. 30, 2018
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. As of September 30, 2018, there were approximately $24.4 million of authorized repurchases remaining. 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. 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 — Starting in fiscal 2016, the Company has a fiscal year that ends on September 30. Prior to fiscal 2016, the fiscal year ended on the last Friday in September. Fiscal year 2018 and 2017 were 52-week fiscal years which ended on September 30, 2018 and September 30, 2017 , respectively. Fiscal year 2016 was a 53-week fiscal year which ended on September 30, 2016 . 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 is generated principally from the sale of its products. Revenue from the sale of products is recognized at the time title, risks and rewards of ownership pass. This is generally when the products reach the free-on-board shipping point, the sales price is fixed and determinable and collection is reasonably assured. The freight charges for shipments are included in the Company's revenues. Revenues are recorded net of tax. Provisions for certain rebates, sales incentives, trade promotions, product returns and discounts to customers are accounted for as reductions in determining sales in the same period the related sales are recorded. Rebates are estimated based on sales terms and historical experience. Product returns are estimated based on historical experience and are recorded at the time revenues are recognized. Accordingly, the Company reduces recognized revenue for estimated future returns at the time revenue is recorded. The estimates for returns are adjusted periodically based upon changes in historical rates of returns and trend analysis. 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 11, ''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 economic 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 2018 and 2017 . 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 350 ("ASC"), "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." The new 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 2018 and fiscal 2016, the Company performed a quantitative impairment assessment for goodwill. The Company calculated the fair value of its 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 operating margins, as well as the perceived risk associated with those forecasts, along with selecting representative 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 2018, 2017 or 2016. 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 difference. 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 developed for purposes of the Company's goodwill impairment test. 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 maintains an indemnity receivable for certain tax obligations that are indemnified by Tyco International Holding S.à.r.l. ("Tyco") and that are expected to be settled directly with the taxing authorities. The Company periodically assesses the realizabilty 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 8, ''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 2017-07 Compensation - Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost The ASU requires an entity to report the service cost component of pension cost and postretirement benefit cost as compensation expense during the employee's service period. The other components of net periodic pension benefit costs will be presented outside a subtotal of income from operations. Prior to the adoption of ASU 2017-07, pension costs were reported as cost of sales and selling, general and administrative expenses on the Company's consolidated statements of income. As a result of the early adoption of ASU 2017-07, the Company reclassified a net credit of $1,505 and $1,453 from operating income to other income, net on the consolidated statements of the years ended September 30, 2017, and September 30, 2016, respectively. 4 2018 2017-09 Compensation - Stock Compensation (Topic 718): Scope of Modification Accounting The ASU does not require an entity to apply modification accounting if the fair value, vesting conditions and classification of the awards do not change. No material impact on the consolidated financial statements. 5 2018 A summary of guidance not yet adopted are as follows: Guidance not yet adopted ASU Description of ASU Impact to Atkore Effective Date 2016-16 Income Taxes (Topic 740): Intra-Entity Transfers of Assets The ASU removes the prohibition in ASC 740 against the immediate recognition of the current and deferred income tax effects of intra-entity transfers of assets other than inventory. Under evaluation 2019 ASU 2018-02 Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income The ASU provided entities the option to reclassify tax effects stranded in accumulated other comprehensive income as a result of the Tax Cuts and Jobs Act to retained earnings. Under evaluation. 2020 2014-09 Revenue from Contracts with Customers The ASU, and subsequent amendments to the guidance issued in fiscal 2018, 2017, 2016 and 2015, provide 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 this standard in the first quarter of 2019 using the modified retrospective method. The adoption of the standard did not have a material impact on the Company’s consolidated financial statements. Based on the completion of its reviews and assessments, the Company has determined that the pattern of revenue recognition for substantially all of its existing revenue streams is substantially unchanged. The Company implemented the appropriate changes to business processes and controls to support recognition and disclosure under the new standard, including the new qualitative and quantitative disclosures that will include information on the nature, amount, timing and significant judgments impacting revenue from contracts with customers. 2019 2016-02 Leases (Topic 842) The ASU, and subsequent amendments to the guidance issued in fiscal 2018 and 2017, require 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. The Company has established an implementation team, deployed lease landscape surveys and is currently evaluating the impact of adoption of this ASU on its consolidated financial statements. 2020 2018-14 Compensation - Retirement Benefits - Defined Benefit Plans - General (Subtopic 715-20) The ASU amends ASC 715 to add, remove and clarify disclosure requirements related to defined benefit pension and other postretirement plans. Under evaluation. 2021 2018-07 Improvements to Nonemployee Share-Based Payment Accounting. ASU 2018-07 simplifies the accounting for share-based payments to nonemployees by aligning it with the accounting for share-based payments to employees, with certain exceptions. Under evaluation. 2020

Acquisitions

Acquisitions	12 Months Ended
	Sep. 30, 2018
Business Combinations [Abstract]
Acquisitions	2. ACQUISITIONS From time to time, the Company enters into strategic acquisitions in an effort to better service existing customers and to attain new customers. 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. 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. 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 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 $24.5 million . The current year adjustments were primarily attributable to a decrease of $18.8 million 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 $2.5 million . Additionally, an adjustment of $8.2 million was recorded for deferred tax liabilities as a result of the purchase accounting adjustments. Goodwill recognized from the Calpipe acquisition is tax-deductible and is amortized over 15 years for income tax purposes. Goodwill recognized from the other acquisitions was non-deductible for tax purposes. The goodwill consists largely of the synergies and economies of scale from integrating these companies with existing businesses. See Note 13, ''Goodwill and Intangible Assets.'' The following table summarizes the fair value of intangible assets acquired in fiscal 2017 as of the acquisition dates: Calpipe Other ($ 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 years ended September 30, 2017 and September 30, 2016 as if the Calpipe acquisition had occurred as of the first day of the fiscal 2016 period: Fiscal Year Ended (in thousands) September 30, 2017 September 30, 2016 Pro forma net sales $ 1,575,801 $ 1,589,136 Pro forma net income 91,362 62,127 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 years ended September 30, 2017 and September 30, 2016 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 non recurring inventory step up adjustment of $2.2 million . 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 2018 and 2017 are included in the consolidated statement of operations for fiscal 2018 and fiscal 2017 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. On September 27, 2018, the Company entered into a definitive agreement with Pentahold NV to acquire Vergokan International NV ("Vergokan"), 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 electrical portfolio of cable management products within the Electrical Raceway segment. On October 1, 2018, the Company acquired all of the outstanding stock of Vergokan for a purchase price of $58,728 . The initial purchase price allocation and pro-forma results of operations have not been completed, as the Company is still determining the fair value estimates of intangible assets, fixed assets and working capital items.

Divestiture

Divestiture	12 Months Ended
	Sep. 30, 2018
Discontinued Operations and Disposal Groups [Abstract]
Divestiture	3. 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 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, 2018
Retirement Benefits [Abstract]
Postretirement Benefits	4. 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. As of September 30, 2018 , 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, 2018 September 30, 2017 September 30, 2016 Service cost $ — $ 2,049 $ 1,894 Interest cost 4,098 3,793 4,143 Expected return on plan assets (6,415 ) (6,601 ) (6,318 ) Amortization of actuarial loss 343 1,303 722 Net periodic (benefit)/cost $ (1,974 ) $ 544 $ 441 The weighted-average assumptions used to determine net periodic pension benefit cost during the period were as follow: September 30, 2018 September 30, 2017 September 30, 2016 Discount rate 3.7 % 3.5 % 4.2 % Expected return on plan assets 6.4 % 7.0 % 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, 2016 $ 133,152 Service cost 2,049 Interest cost 3,793 Actuarial (gain) (5,012 ) Benefits and administrative expenses paid (5,130 ) Balance as of September 30, 2017 128,852 Service cost — Interest cost 4,098 Actuarial (gain) (7,130 ) Benefits and administrative expenses paid (4,863 ) Balance as of September 30, 2018 $ 120,957 Change in plan assets: Balance as of September 30, 2016 $ 97,980 Actual return on plan assets 10,388 Employer contributions 375 Benefits and administrative expenses paid (5,130 ) 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 Funded status as of September 30, 2018 $ (16,503 ) (in thousands) September 30, 2018 September 30, 2017 Amounts recognized in the consolidated balance sheets consist of: Pension assets $ 572 $ — Pension liabilities (17,075 ) (25,239 ) Net amount recognized $ (16,503 ) $ (25,239 ) Amounts recognized in accumulated other comprehensive loss (before income taxes) consist of: Net actuarial loss $ (12,344 ) $ (18,103 ) Total loss recognized $ (12,344 ) $ (18,103 ) Weighted-average assumptions used to determine pension benefit obligations at year end: Discount rate 4.2 % 3.7 % 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, 2018 September 30, 2017 Accumulated benefit obligation $ 105,414 $ 128,852 Fair value of plan assets 88,339 103,613 The following table summarizes the defined benefit pension plans with projected benefit obligations in excess of plan assets: (in thousands) September 30, 2018 September 30, 2017 Projected benefit obligation $ 105,414 $ 128,852 Fair value of plan assets 88,339 103,613 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, 2018 September 30, 2017 Equity securities 59% 61% Debt securities 39% 38% Cash and cash equivalents 2% 1% 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, 2018 , 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, 2018 September 30, 2017 Level 1 Level 2 Total Level 1 Level 2 Total United States equity securities $ 32,447 $ 6,354 $ 38,801 $ — $ 36,362 $ 36,362 Non-U.S. equity securities 20,164 2,599 22,763 — 26,432 26,432 Fixed income securities 34,111 6,678 40,789 — 39,319 39,319 Cash and cash equivalents 2,101 — 2,101 1,500 — 1,500 Total $ 88,823 $ 15,631 $ 104,454 $ 1,500 $ 102,113 $ 103,613 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, 2018 and September 30, 2017 , 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 $1,003 and $375 to its pension plans for the fiscal years ended September 30, 2018 and September 30, 2017 . The Company anticipates that it will contribute at least the minimum required contribution of $1,077 to its pension plans in fiscal 2019 . Benefit payments, which reflect future expected service as appropriate, are expected to be paid in each fiscal year as follows: (in thousands) 2019 $ 5,717 2020 6,011 2021 6,340 2022 6,609 2023 6,814 2024 and thereafter 36,690 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 $3,600 , $3,000 and $2,817 for the fiscal years ended September 30, 2018 , September 30, 2017 and September 30, 2016 , respectively. Multi-Employer Plan — The Company has a liability of $5,934 as of September 30, 2018 and $6,250 as of September 30, 2017 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, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock Incentive Plan	5. 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 3.8 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 $14,664 , $12,788 and $21,127 for fiscal years 2018 , 2017 and 2016 , respectively. The total income tax benefit recognized for share-based compensation arrangements was $3,681 , $4,731 and $7,704 for fiscal years 2018 , 2017 and 2016 , 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 On September 26, 2014, the Company's Board of Directors modified the Stock Incentive Plan to provide the Company with the discretion to net settle stock option awards in cash upon exercises by holders upon termination of their employment. Starting at that time, options were considered liability awards and thus were measured and recorded at their estimated fair value. The Company utilized equity valuations based on comparable publicly-traded companies, discounted free cash flows, an analysis of the Company's enterprise value and any other factors deemed relevant in estimating the fair value of the common stock. The fair values of outstanding options were remeasured each reporting period using the Black-Scholes model. Outstanding options granted under the Stock Incentive Plan vested ratably over five years. Outstanding options granted under the Omnibus Incentive Plan vest ratably over three years. On July 27, 2016 , the Company's Board of Directors modified the Stock Incentive Plan and terminated the net settlement feature, thereby requiring the stock option holders to exercise in an open market transaction through broker-assisted sales. The modification, which affected 63 employees, triggered a change from liability accounting to equity accounting. Consequently, the Company reclassified $43,870 from non-current liabilities to additional-paid-in-capital. The Company marked the options to their fair value as of July 27, 2016 using the Black-Scholes option pricing model resulting in $2.4 million of additional expense for vested 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, 2018 September 30, 2017 September 30, 2016 Expected dividend yield — % — % — % Expected volatility 40 % 40 % 40 % Range of risk-free interest rates (%) 2.16-2.38 1.95 0.74 - 1.27 Range of expected option lives 1.00 to 6.00 years 6.00 years 2.09 - 6.37 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 25, 2015 to September 30, 2018 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 25, 2015 6,746 $ 7.70 Granted 72 15.79 $ 7.63 Exercised (18 ) 7.95 $ 43 Forfeited (136 ) 8.84 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 $ 43,302 5.61 Exercisable as of September 30, 2018 1,915 $ 8.41 $ 34,697 5.04 As of September 30, 2018 , there was $2,911 of total unrecognized compensation expense related to non-vested options granted expected to be recognized over a weighted-average period of approximately 1.36 years. The total fair value of shares vested during fiscal years 2018 , 2017 and 2016 was $9,757 , $10,696 and $12,507 , respectively. Cash received from stock option exercises for the fiscal years 2018 , 2017 and 2016 was $20,110 , $12,168 and $52 , respectively. The actual tax benefit for the tax deductions from stock option exercises totaled $11,478 , $9,530 and $19 , respectively, for fiscal years 2018 , 2017 and 2016 . The amount of cash the Company paid to settle the options exercised during the fiscal years ended 2018 , 2017 and 2016 was $0 , $0 and $43 , respectively. 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, 2018 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 As of September 30, 2018 , there was $ 7,550 of total unrecognized compensation expense related to non-vested RSUs granted, expected to be recognized over a weighted-average period of approximately 1.9 years. The total fair value of RSUs vested during fiscal years 2018 , 2017 and 2016 was $ 1,969 , $0 and $300 , respectively. Performance Share Units On November 30, 2016 and November 28, 2017, 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 2018 and 2017, the closing stock price on the date of grant was $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, 2018 were as follows: September 30, 2018 September 30, 2017 Expected dividend yield — % — % Range of expected volatility 17.21 - 79.15 17.55 - 75.55 Risk free interest rates 1.83 - 2.54 1.35 % Expected life 1 - 3 years 3 years Fair value $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, 2018 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 As of September 30, 2018 , there was $4,472 of total unrecognized compensation expense related to non-vested PSUs granted, expected to be recognized over a weighted-average period of approximately 1.87 years. There were no PSUs vested during fiscal years 2018 , 2017 and 2016 .

Restructuring Charges and Asset

Restructuring Charges and Asset Impairments	12 Months Ended
	Sep. 30, 2018
Restructuring and Related Activities [Abstract]
Restructuring Charges and Asset Impairments	6. RESTRUCTURING CHARGES AND ASSET IMPAIRMENTS 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 . See Note 1, ''Basis of Presentation and Summary of Significant Accounting Policies.'' Electrical Raceway Mechanical Products & Solutions Other/Corporate (in thousands) Severance (a) Other (a) Severance (b) Other (b) Severance Other Total Balance as of September 25, 2015 $ 206 $ — $ 3,511 $ 620 $ 15 $ 61 $ 4,413 Charges 807 — 689 2,583 — 199 4,278 Utilization (168 ) — (4,017 ) (2,542 ) (11 ) (260 ) (6,998 ) Reversals — — (183 ) (122 ) (4 ) — (309 ) Exchange rate effects $ (4 ) $ — $ — $ — $ — $ — $ (4 ) 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 (a) Primarily related to Atkore's commitment to close certain facilities as part of its continuing effort to realign its strategic focus. The Company recorded severance restructuring charges of $240 , $297 and $778 related to termination benefits during the fiscal years ended 2018, 2017 and 2016, respectively. The Company recorded other restructuring charges to close facilities of $976 and $297 for the fiscal years ending September 30, 2018 and September 30, 2017 , respectively. (b) Primarily related to the August 6, 2015 announcement of the Company's plans to exit its Fence and Sprinkler steel pipe and tube product lines ("Fence and Sprinkler") in order to realign its long-term strategic focus. The operations associated with these product lines were wound down during the first quarter of fiscal 2016 and resulted in headcount reductions in the Company's Philadelphia, Phoenix and Harvey facilities. The Company recorded $630 of severance-related expenses for the fiscal year ended September 30, 2016 related to the headcount reductions. The Company also recorded $64 and $2,066 of facility-related and other charges related to the closure of the facilities during fiscal 2017 and 2016 respectively. The Company expects to utilize all restructuring accruals as of September 30, 2018 within the next twelve months. The net restructuring charges included as a component of selling, general and administrative expense and asset impairment charges in the Company's consolidated statements of operations were as follows: Fiscal Year Ended (in thousands) September 30, 2018 September 30, 2017 September 30, 2016 Total restructuring charges, net 1,849 1,256 3,967 Asset impairment charges — — 129 Fence and Sprinkler Asset Impairment Charges During fiscal 2016 , the Company recorded impairment charges of $129 for the write-down of prepaid shop supplies related to Fence and Sprinkler.

Other Income, Net

Other Income, Net	12 Months Ended
	Sep. 30, 2018
Other Income, Nonoperating [Abstract]
Other Income, Net	7. OTHER INCOME, NET Other income, net consisted of the following: Fiscal Year Ended Fiscal Year Ended Fiscal Year Ended (in thousands) September 30, 2018 September 30, 2017 As Adjusted** September 30, 2016 As Adjusted** Gain on sale of a business $ (27,575 ) $ — $ — Gain on sale of joint venture — (5,774 ) — Undesignated foreign currency derivative instruments (121 ) 2,741 — Foreign exchange gain (loss) on intercompany loans 1,500 (2,038 ) — Debt modification costs 892 — — Pension-related benefits (1,975 ) (1,504 ) (1,452 ) Other (69 ) — — Other income, net $ (27,348 ) $ (6,575 ) $ (1,452 ) ** As adjusted due to the adoption of ASU 2017-07. See Note 1, ''Basis of Presentation and Summary of Significant Accounting Policies'' for additional information.

Income Taxes

Income Taxes	12 Months Ended
	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	8. INCOME TAXES On December 22, 2017, "H.R.1," also known as the Tax Cuts and Jobs Act ("TCJA") was signed into law. This newly enacted legislation provides 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 Company has estimated the impact of the new legislation on its financial position based on information currently available and will continue to assess the impact as additional guidance is received. As a fiscal year filer, the Company has a blended federal statutory rate of 24.5% for fiscal year 2018, in accordance with rules described in Section 15 of the Internal Revenue Code, and a federal statutory rate of 21.0% for fiscal year 2019 and following years. 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 period of enactment. At September 30, 2018, the Company has recorded an adjustment to the re-measurement of deferred tax liabilities of an additional $708 benefit as a result of updated estimates. The Company has an accumulated earnings and profits deficit in the foreign jurisdictions in which it operates. As a result, it does not anticipate an income tax liability from the one-time transition tax on the deemed repatriation of its foreign earnings. The tax impact of the new legislation was based on the Company's best estimate. The provisional amounts incorporate assumptions made based upon the Company's current interpretation of the TCJA and may change as the Company receives additional clarification and implementation guidance. Further adjustments to the provisional amount will be included in income from continuing operations as an adjustment to income tax expense on the Company's consolidated statements of operations through the end of the measurement period in the first quarter of fiscal 2019. Significant components of income before income taxes and income tax expense for the fiscal years ended September 30, 2018 , September 30, 2017 and September 30, 2016 consisted of the following: (in thousands) September 30, 2018 September 30, 2017 September 30, 2016 Components of income before income taxes: United States $ 155,453 $ 116,580 $ 78,016 Non-U.S 10,899 9,545 8,765 Income before income taxes $ 166,352 $ 126,125 $ 86,781 Income tax expense: Current: United States: Federal $ 26,586 $ 33,127 $ 18,748 State 8,662 4,246 4,655 Non-U.S: 3,467 3,175 2,026 Current income tax expense $ 38,715 $ 40,548 $ 25,429 Deferred: United States: Federal $ (7,149 ) $ 224 $ 642 State (1,793 ) 469 1,872 Non-U.S: (66 ) 245 42 Deferred income tax (benefit) expense (9,008 ) 938 2,556 Income tax expense $ 29,707 $ 41,486 $ 27,985 In addition to the change in the U.S. federal statutory rate, t he mix of foreign losses and domestic losses, along with rate reconciling items as outlined below, impacts the effective tax rate for the periods. Differences between the statutory federal income tax rate and effective income tax rate are summarized below: (in thousands) September 30, 2018 September 30, 2017 September 30, 2016 Statutory federal tax 25 % 35 % 35 % Adjustments to reconcile to the effective income tax rate: State income taxes 3 % 3 % 5 % Nondeductible expenses — % — % 2 % Change in tax rates (3 )% — % — % Domestic manufacturing deduction (2 )% (2 )% (3 )% Indemnified uncertain tax benefits (2 )% — % (5 )% Stock-based compensation (3 )% (3 )% — % Other — % — % (2 )% Effective income tax rate 18 % 33 % 32 % 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. 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. The Company's effective tax rate for fiscal 2016 differs from the statutory rate primarily due to a $4,332 tax benefit from the release of indemnified uncertain tax positions, a $2,805 tax benefit for domestic manufacturing deduction, partially offset by $4,625 of state income tax expense and $1,685 of non-deductible transaction costs. 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, 2018 September 30, 2017 Deferred tax assets: Accrued liabilities and reserves $ 32,099 $ 48,619 Tax loss and credit carryforwards 12,435 15,340 Postretirement benefits 4,317 9,863 Inventory 11,220 6,482 Other 218 877 $ 60,289 $ 81,181 Deferred tax liabilities: Property, plant and equipment $ (13,739 ) $ (13,770 ) Intangible assets (48,375 ) (65,072 ) Loss on investment (5,321 ) (6,085 ) Other (1,775 ) (2,549 ) $ (69,210 ) $ (87,476 ) Net deferred tax liability before valuation allowance (8,921 ) (6,295 ) Valuation allowance (7,427 ) (9,512 ) Net deferred tax liability $ (16,348 ) $ (15,807 ) As of September 30, 2018 , the Company has $43,312 of state net operating loss carryforwards which expire beginning in 2018 through 2035. In certain non-U.S. jurisdictions, the Company has net operating loss carryforwards of $38,434 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 2018, the Company reassessed the need for a valuation allowance against deferred tax assets in the Company's Asia Pacific business based on recent losses and the expected future performance of the business. As a result, the Company has reestablished a $600 valuation allowance against the deferred tax assets of the Asia Pacific business. As of September 30, 2018 , the Company had unrecognized tax benefits of $1,364 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, 2018 and September 30, 2017 , the Company had accrued interest and penalties of $78 and $3,193 , respectively, in the consolidated balance sheets. A reconciliation of the beginning and ending amount of unrecognized tax benefit, excluding interest and penalties, is as follows: (in thousands) For the period from September 30, 2016 to September 30, 2018 Balance as of September 25, 2015 $ 8,101 Additions based on tax positions related to prior years 62 Settlements (4,360 ) 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 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 . During fiscal 2016 , the balance of unrecognized tax benefits decreased by $4,360 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 $596 . 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 2014, 2016, and 2017 The Company's income tax returns are examined periodically by various taxing authorities. The Company's federal tax return for fiscal 2015 was examined and completed in fiscal year 2017, with no change as a result of the examination. 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. During 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, 2018 , certain subsidiaries had approximately $39,978 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. Accordingly, the Company has reflected those liabilities with an offsetting receivable due from Tyco of $122 on the consolidated balance sheet as of September 30, 2018 . If the Company's estimate of uncertain tax liabilities arising in the periods following the Transactions proves to be less than the ultimate assessment, an additional charge to expense would result. If payment of these amounts ultimately proves to be less than the recorded amounts, the reversal of the liabilities may result in income tax benefits being recognized in the period when the Company determines the liabilities are no longer necessary. 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, 2018
Earnings Per Share [Abstract]
Earnings Per Share	9. 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, 2018 September 30, 2017 September 30, 2016 Numerator: Net income $ 136,645 $ 84,639 $ 58,796 Less: Undistributed earnings allocated to participating securities 2,456 241 32 Net income available to common shareholders $ 134,189 $ 84,398 $ 58,764 Denominator: Basic weighted average common shares outstanding 51,791 63,420 62,486 Effect of dilutive securities: Non-participating employee stock options (1) 2,298 3,134 300 Diluted weighted average common shares outstanding 54,089 66,554 62,786 Basic earnings per share $ 2.59 $ 1.33 $ 0.94 Diluted earnings per share $ 2.48 $ 1.27 $ 0.94 (1) Stock options to purchase approximately 0.3 million , 0.2 million , and 0.1 million shares of common stock were outstanding during the years ended September 30, 2018 , September 30, 2017, and September 30, 2016, 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, 2018
Equity [Abstract]
Accumulated Other Comprehensive Loss	10. 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, 2016 $ (17,192 ) $ (8,758 ) $ (25,950 ) Other comprehensive loss before reclassifications 5,941 1,221 7,162 Amounts reclassified from accumulated other comprehensive loss 806 — 806 Net current period other comprehensive loss 6,747 1,221 7,968 Balance as of September 30, 2017 $ (10,445 ) $ (7,537 ) $ (17,982 ) Other comprehensive income 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 ) The following is a summary of the amounts reclassified from accumulated other comprehensive loss to net income : Fiscal Year Ended (in thousands) September 30, 2018 September 30, 2017 September 30, 2016 Amortization of defined benefit pension items: Amortization of net loss (included within other income, net) $ 343 $ 1,303 $ 722 Tax expense (14 ) (497 ) (276 ) Net reclassifications for the period $ 329 $ 806 $ 446 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 $101 .

Inventories, Net

Inventories, Net	12 Months Ended
	Sep. 30, 2018
Inventory Disclosure [Abstract]
Inventories, Net	11. INVENTORIES, NET A majority of the Company records inventory at the lower of cost or market (primarily last in, first out, or "LIFO"). Approximately 80% and 75% of the Company's inventories are valued at the lower of LIFO cost or market at September 30, 2018 and September 30, 2017 , respectively. (in thousands) September 30, 2018 September 30, 2017 Purchased materials and manufactured parts, net $ 58,572 $ 49,168 Work in process, net 21,769 17,598 Finished goods, net 141,412 133,237 Inventories, net $ 221,753 $ 200,003 Total inventories would be $26,340 and $4,915 higher than reported as of September 30, 2018 and September 30, 2017 , respectively, if the first-in, first-out method was used for all inventories. As of September 30, 2018 and September 30, 2017 , the excess and obsolete inventory reserve was $12,909 and $8,432 , respectively.

Property, Plant and Equipment

Property, Plant and Equipment	12 Months Ended
	Sep. 30, 2018
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	12. PROPERTY, PLANT AND EQUIPMENT As of September 30, 2018 and September 30, 2017 , property, plant and equipment at cost and accumulated depreciation were as follows: (in thousands) September 30, 2018 September 30, 2017 Land $ 13,295 $ 13,296 Buildings and related improvements 108,758 105,154 Machinery and equipment 262,078 248,440 Leasehold improvements 7,382 6,744 Software 30,502 15,135 Construction in progress 16,777 16,160 Property, plant and equipment 438,792 404,929 Accumulated depreciation (225,684 ) (196,310 ) Property, plant and equipment, net $ 213,108 $ 208,619 Depreciation expense for fiscal years ended September 30, 2018 , September 30, 2017 and September 30, 2016 totaled $34,786 , $32,320 and $32,779 , respectively.

Goodwill and Intangible Assets

Goodwill and Intangible Assets	12 Months Ended
	Sep. 30, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets	13. 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, 2016 $ 76,640 $ 39,189 $ 115,829 Goodwill acquired during year 31,477 — $ 31,477 Exchange rate effects 410 — $ 410 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 balances include $3,924 and $43,000 of accumulated impairment losses within the Electrical Raceway and MP&S segments, respectively, as of September 30, 2018 and September 30, 2017. 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, 2018 September 30, 2017 (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 $ 330,295 $ (141,401 ) $ 188,894 $ 350,129 $ (118,273 ) $ 231,856 Other 8 16,003 (5,861 ) 10,142 27,819 (9,266 ) 18,553 Total 346,298 (147,262 ) 199,036 377,948 (127,539 ) 250,409 Indefinite-lived Intangible Assets: Trade names 92,880 — 92,880 93,880 — 93,880 Total $ 439,178 $ (147,262 ) $ 291,916 $ 471,828 $ (127,539 ) $ 344,289 Amortization expense for the fiscal years ended September 30, 2018 , September 30, 2017 and September 30, 2016 was $32,104 , $22,407 and $22,238 , respectively. Expected amortization expense for intangible assets over the next five years and thereafter is as follows (in thousands) : 2019 $ 31,122 2020 29,401 2021 29,267 2022 27,981 2023 27,860 2024 and thereafter 53,405 Actual amounts of amortization may differ from estimated amounts due to additional intangible asset acquisitions, changes in estimated useful lives, impairment of intangible assets, and other events.

Debt

Debt	12 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Debt	14. DEBT Debt as of September 30, 2018 and September 30, 2017 was as follows: (in thousands) September 30, 2018 September 30, 2017 First Lien Term Loan Facility due December 22, 2023 $ 912,162 $ 495,134 ABL Credit Facility — 85,000 Deferred financing costs (8,194 ) (4,496 ) Other 279 440 Total debt $ 904,247 $ 576,078 Less: Current portion (a) 26,561 4,215 Long-term debt $ 877,686 $ 571,863 (a) Of the amount due within 2019 for the First Lien Term Loan Facility, $18,936 relates to an accelerated repayment of principal calculated by a formula based on 2018 excess cash flows and a leverage ratio as defined within the Term Loan Agreement. No such accelerated payments are indicated for any of the periods beyond 2019. As of September 30, 2018 , future contractual maturities of long-term debt are as follows (in thousands) : 2019 (a) $ 28,136 2020 9,200 2021 9,200 2022 9,200 2023 9,200 2024 and thereafter $ 848,164 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 January 22, 2016 , AII redeemed $17,000 of the Second Lien Term Loan Facility at a redemption price of 89.00% of the par value, and $2,000 at a redemption price of 89.75% of the par value. For the fiscal year ended September 30, 2016, the Company recorded a gain on the extinguishment of debt of $1,661 . 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 million , 2) repay $42.0 million of outstanding loans under the ABL Credit Facility 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 $315,119 and $172,994 as of September 30, 2018 and September 30, 2017 , 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 $0 and $85 million of borrowings outstanding under the ABL Credit Facility as of September 30, 2018 and September 30, 2017 , respectively. The company paid a fee of 0.50% on unutilized availability in fiscal 2016 and up until December 22, 2016, and a fee of 0.375% for the period from December 22, 2016 through September 30, 2018. 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. The First Lien Term Loan Facility and the ABL Credit Facility are secured by all of the assets of the U.S. subsidiaries of the Company. The First Lien Term Loan Facility has priority over all real property, plant and equipment, intellectual property and capital stock of any US subsidiary and any documents or instruments evidencing the foregoing assets. The ABL Credit Facility has first priority over cash and cash equivalents, accounts receivable, inventory and other documents and instruments evidencing the foregoing assets.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value Measurements	15. 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 six years. Short-term forward currency contracts are recorded in other current liabilities and long-term forward currency contracts are recorded in 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 7, ''Other Income, net'' for further detail. The total notional amounts of undesignated forward currency contracts were £ 49.1 million and £ 52.6 million as of September 30, 2018 and September 30, 2017 , respectively. There were no undesignated forward currency contracts as of September 30, 2016 . 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, 2018 and September 30, 2017 in accordance with the fair value hierarchy: September 30, 2018 September 30, 2017 (in thousands) Level 1 Level 2 Level 3 Level 1 Level 2 Level 3 Assets Cash equivalents $ 28,175 $ — $ — $ 571 $ — $ — Liabilities Forward currency contracts — $ 1,857 — — $ 2,936 — In addition to the recurring assets and liabilities measured at fair value, the Company has a contingent consideration liability of $650 measured at fair value related to the Cii acquisition, which will be settled in the second quarter of fiscal 2019. The liability is valued using significant unobservable inputs (Level 3) based on the probability of achievement of first year earnings post acquisition. See Note 2, ''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, 2018 September 30, 2017 (in thousands) Carrying Value Fair Value Carrying Value Fair Value First Lien Term Loan Facility due December 22, 2023 $ 913,100 $ 916,113 $ 496,250 $ 498,979 Total debt $ 913,100 $ 916,113 $ 496,250 $ 498,979 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, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	16. COMMITMENTS AND CONTINGENCIES The Company has obligations related to commitments to purchase certain goods. As of September 30, 2018 , such obligations were $100,649 for fiscal 2019 , $330 for fiscal 2020 and $46 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 $17,403 , $12,941 and $11,934 in fiscal 2018 , 2017 , and 2016 , respectively. At September 30, 2018 , minimum future operating lease payments for leases with terms in excess of one year are presented in the table below as follows (in thousands): 2019 $ 11,069 2020 8,226 2021 6,580 2022 4,886 2023 4,254 2024 and thereafter 9,988 Total $ 45,003 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 $6,755 and $5,872 as of September 30, 2018 and September 30, 2017 , respectively. As of September 30, 2018 , the Company believes that the range of probable losses for Special Products Claims and other product liabilities is between $3,000 and $10,000 . After September 30, 2018, 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 million , as opposed to the $13 million cap negotiated within the original indemnity agreement. Tyco and the Company are examining the Company's total Special Products Claim payments which are nearly $12 million with the aforementioned settlement payment and as of September 30, 2018, the Company believes that it has recorded $12 million of cumulative expense related to these Special Products Claims. From and after the point where the Company has spent $12 million on Special Products Claims, Tyco, now JCI, has a contractual obligation to indemnify the Company in respect of claims of incompatibility between the Company's antimicrobial coated steel sprinkler pipe and CPVC pipe used in the same sprinkler system. 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 subjects 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 the Company has 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, 2018
Guarantees [Abstract]
Guarantees	17. GUARANTEES The Company has outstanding letters of credit totaling $9.9 million 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 $25.3 million as of September 30, 2018 . 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. In the normal course of business, the Company is liable for product performance and contract completion. In the opinion of management, such obligations will not 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, 2018
Related Party Transactions [Abstract]
Related Party Transactions	18. 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 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 no longer owned any of the company's common stock as of September 30, 2018 . Prior to the CD&R Investor selling all of its remaining shares of the Company in May 2018, transactions between the Company, CD&R and affiliates of CD&R were considered to be between related parties. In addition to the consulting fees referenced above, affiliates of CD&R own equity positions in one of the Company's customers. Net sales during fiscal 2018 and 2017 to this customer were $6.0 and $8.3 million , respectively. There were no related party transactions during fiscal 2016 .

Segment Information

Segment Information	12 Months Ended
	Sep. 30, 2018
Segment Reporting [Abstract]
Segment Information	19. 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. Prior to fiscal 2017, income (loss) before income taxes was also adjusted to exclude net periodic pension benefit cost and routine Special Products Claims . Beginning in fiscal 2017, these costs are no longer excluded. Prior fiscal years have not been revised for this change due to the relative insignificance and nature of these amounts. 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, 2018 September 30, 2017 September 30, 2016 (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 $ 1,365,067 $ 1,544 $ 255,260 $ 1,093,500 $ 1,283 $ 189,351 $ 1,066,711 $ 1,919 $ 181,939 MP&S 470,072 81 $ 51,339 410,434 98 $ 63,687 456,673 148 $ 81,199 Eliminations — (1,625 ) — (1,381 ) — (2,067 ) Consolidated operations $ 1,835,139 $ — $ 1,503,934 $ — $ 1,523,384 $ — Capital Expenditures Total Assets (in thousands) September 30, 2018 September 30, 2017 September 30, 2016 September 30, 2018 September 30, 2017 September 30, 2016 Electrical Raceway $ 16,389 $ 13,037 $ 9,161 $ 751,024 $ 757,775 $ 566,250 MP&S 14,267 8,212 6,130 291,164 306,229 343,002 Unallocated 7,845 3,873 1,539 281,872 151,088 255,316 Consolidated operations $ 38,501 $ 25,122 $ 16,830 $ 1,324,060 $ 1,215,092 $ 1,164,568 Presented below is a reconciliation of operating segment Adjusted EBITDA to Income before income taxes : Fiscal Year Ended (in thousands) September 30, 2018 September 30, 2017 September 30, 2016 Operating segment Adjusted EBITDA Electrical Raceway $ 255,260 $ 189,351 $ 181,939 MP&S 51,339 63,687 81,199 Total $ 306,599 $ 253,038 $ 263,138 Unallocated expenses (a) (35,050 ) (25,430 ) (28,136 ) Depreciation and amortization (66,890 ) (54,727 ) (55,017 ) Interest expense, net (40,694 ) (26,598 ) (41,798 ) Gain (loss) on extinguishment of debt — (9,805 ) 1,661 Restructuring & impairments (1,849 ) (1,256 ) (4,096 ) Net periodic pension benefit cost — — (441 ) Stock-based compensation (14,664 ) (12,788 ) (21,127 ) Special products claims impact — — (850 ) Certain legal matters 4,833 (7,551 ) (1,382 ) Consulting fees — — (15,425 ) Transaction costs (9,314 ) (4,779 ) (7,832 ) Gain on sale of a business 27,575 — — Gain on sale of joint venture — 5,774 — Other (b) (4,194 ) 10,247 (1,103 ) Impact of Fence and Sprinkler exit — — (811 ) Income before income taxes $ 166,352 $ 126,125 $ 86,781 (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, 2018 September 30, 2017 September 30, 2016 September 30, 2018 September 30, 2017 September 30, 2016 United States $ 201,101 $ 202,823 $ 204,640 $ 1,651,636 $ 1,367,907 $ 1,395,750 Other Americas 138 164 175 43,013 37,908 40,573 Europe 11,090 9,306 1,295 90,915 55,181 40,246 Asia-Pacific 2,386 3,378 3,826 49,575 42,938 46,815 Total $ 214,715 $ 215,671 $ 209,936 $ 1,835,139 $ 1,503,934 $ 1,523,384 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, 2018 September 30, 2017 September 30, 2016 Metal Electrical Conduit and Fittings $ 517,935 $ 349,239 $ 331,526 Armored Cable and Fittings 336,388 323,070 318,279 PVC Electrical Conduit & Fittings 311,811 265,389 258,954 Other raceway products 198,933 155,802 157,952 Electrical Raceway 1,365,067 1,093,500 1,066,711 Mechanical Pipe 253,381 211,245 249,473 Other MP&S products 216,691 199,189 199,384 Impact of Fence and Sprinkler — — 7,816 MP&S 470,072 410,434 456,673 Net sales $ 1,835,139 $ 1,503,934 $ 1,523,384 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, 2018, one customer, Sonepar Management US, Inc., represented 11% of the Company's accounts receivable balance due to increased sales in the last 60 days of the year. As of 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 and 2016, no single customer accounted for more than 10% of sales or accounts receivable. Concentration of Employees — As of September 30, 2018 , approximately 26% 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 12 employees, expired on November 11, 2018. We anticipate that the new collective bargaining agreement will be ratified by the union members within calendar 2018. Our Harvey Illinois collective bargaining agreement with the United Steelworkers, involving nearly 400 represented employees, does not expire until April 2020. The Company believes its relationship with their employees is good.

Quarterly Operating Results (Un

Quarterly Operating Results (Unaudited)	12 Months Ended
	Sep. 30, 2018
Quarterly Financial Information Disclosure [Abstract]
Quarterly Operating Results (Unaudited)	20. QUARTERLY OPERATING RESULTS (UNAUDITED) The following table presents unaudited selected quarterly financial data for the years ended September 30, 2018 and September 30, 2017 . 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 29, 2017 March 30, 2018 June 29, 2018 September 30, 2018 December 30, 2016 As Adjusted ** (1) March 31, 2017 As Adjusted ** (2) June 30, 2017 As Adjusted ** September 30, 2017 As Adjusted ** Net sales $ 414,558 $ 445,000 $ 498,014 $ 477,567 $ 337,591 $ 372,791 $ 397,745 $ 395,807 Gross profit 96,867 109,157 120,329 111,731 91,665 87,609 92,485 89,513 Net income 27,189 42,558 34,199 32,699 17,382 18,935 27,465 20,857 Net income per share Basic (6) $ 0.43 $ 0.83 $ 0.73 $ 0.69 $ 0.28 $ 0.30 $ 0.43 $ 0.33 Diluted (6) 0.41 0.79 0.70 0.66 0.26 0.28 0.41 0.31 ** As adjusted due to the adoption of ASU 2017-07. See Note 1, ''Basis of Presentation and Summary of Significant Accounting Policies'' for additional information. (1) Includes a $9,805 loss on the extinguishment of the Initial First Lien Term Loan Facility and the Second Lien Term Loan Facility. See Note 14, "Debt." (2) Includes $5,774 pre-tax gain on sale of Abahsain-Cope Saudi Arabia Ltd joint venture and $7,501 pre-tax expense related to the Antidumping Duty Order for Malleable Iron Pipe Fittings. See Note 16, "Commitments and contingencies" for additional information (3) Includes a $4,758 benefit due to the re-measurement of deferred tax liabilities as a result of the TCJA. See Note 8, "Income taxes" for additional information. (4) 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 3, "Divestitures" for additional information. (5) Includes $7,171 pre- tax reversal of expense related to the Antidumping Duty Order for Malleable Iron Pipe Fittings. See Note 16, Commitments and contingencies" for additional information. (6) 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, 2018
Condensed Financial Information of Parent Company Only 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, 2018 September 30, 2017 Assets Investment in subsidiary $ 122,059 $ 360,871 Total Assets 122,059 360,871 Liabilities and Equity Total Liabilities $ — $ — Equity: Common stock, $0.01 par value, 1,000,000,000 shares authorized, 47,079,645 and 63,305,434 shares issued and outstanding, respectively $ 472 $ 634 Treasury stock, held at cost, 260,900 and 260,900 shares, respectively (2,580 ) (2,580 ) Additional paid-in capital 457,978 423,232 Accumulated deficit (317,373 ) (42,433 ) Accumulated other comprehensive loss (16,438 ) (17,982 ) Total Equity 122,059 360,871 Total Liabilities and Equity $ 122,059 $ 360,871 See Notes to Financial Statements SCHEDULE I ATKORE INTERNATIONAL GROUP INC. (PARENT) CONDENSED FINANCIAL INFORMATION CONDENSED STATEMENTS OF OPERATIONS Fiscal Year Ended (in thousands) September 30, 2018 September 30, 2017 September 30, 2016 Equity in net income of subsidiary $ 136,645 $ 84,639 $ 58,796 Net income 136,645 84,639 58,796 Other comprehensive income (loss) of subsidiary, net of tax 1,544 7,968 (4,917 ) Comprehensive income $ 138,189 $ 92,607 $ 53,879 See Notes to Financial Statements SCHEDULE I ATKORE INTERNATIONAL GROUP INC. (PARENT) CONDENSED FINANCIAL INFORMATION CONDENSED STATEMENTS OF CASH FLOWS For the Year Ended (in thousands) September 30, 2018 September 30, 2017 September 30, 2016 Cash Flows from Operating Activities: Net cash provided by operating activities $ — $ — $ — Cash Flows from Investing Activities: Distribution received from subsidiary 411,775 13,938 — Distribution paid to subsidiary (20,110 ) (12,168 ) (52 ) Net cash provided by (used in) investing activities 391,665 1,770 (52 ) Cash Flows from Financing Activities: Issuance of common shares 20,110 12,168 52 Repurchase of common shares (411,775 ) (13,938 ) — Net cash (used in) provided by financing activities (391,665 ) (1,770 ) 52 Net change in cash and cash equivalents — — — Cash and cash equivalents: Beginning — — — Ending $ — $ — $ — See Notes to Financial Statements SCHEDULE I ATKORE INTERNATIONAL GROUP INC. (PARENT) CONDENSED FINANCIAL INFORMATION NOTES TO CONDENSED FINANCIAL STATEMENTS (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 $411,775 , $13,938 , and $0 from its subsidiaries for the years ended September 30, 2018 , September 30, 2017 and September 30, 2016 , respectively. The distributions received in fiscal 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. 4. Common Stock Split On May 27, 2016, the Company filed a Certificate of Amendment to amend and restate the Company's Certificate of Incorporation in the State of Delaware, effecting a 1.37 -for-1 common stock split. All applicable share data, per share amounts and related information in the consolidated financial statements and notes thereto have been adjusted retroactively to give effect to the 1.37 -for-1 common stock split.

Schedule II - Valuation and Qua

Schedule II - Valuation and Qualifying Accounts	12 Months Ended
	Sep. 30, 2018
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: 2018 $ (1,239 ) (389 ) (134 ) $ (1,762 ) 2017 $ (1,006 ) (243 ) 10 $ (1,239 ) 2016 $ (1,173 ) (426 ) 593 $ (1,006 ) Deferred Tax Valuation Allowance: For the fiscal year ended: 2018 $ (9,512 ) 428 1,657 $ (7,427 ) 2017 $ (8,658 ) (1,164 ) 310 $ (9,512 ) 2016 $ (7,532 ) (2,604 ) 1,478 $ (8,658 )

Basis of Presentation and Sum_2

Basis of Presentation and Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Sep. 30, 2018
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 — Starting in fiscal 2016, the Company has a fiscal year that ends on September 30. Prior to fiscal 2016, the fiscal year ended on the last Friday in September. Fiscal year 2018 and 2017 were 52-week fiscal years which ended on September 30, 2018 and September 30, 2017 , respectively. Fiscal year 2016 was a 53-week fiscal year which ended on September 30, 2016 . 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 is generated principally from the sale of its products. Revenue from the sale of products is recognized at the time title, risks and rewards of ownership pass. This is generally when the products reach the free-on-board shipping point, the sales price is fixed and determinable and collection is reasonably assured. The freight charges for shipments are included in the Company's revenues. Revenues are recorded net of tax. Provisions for certain rebates, sales incentives, trade promotions, product returns and discounts to customers are accounted for as reductions in determining sales in the same period the related sales are recorded. Rebates are estimated based on sales terms and historical experience. Product returns are estimated based on historical experience and are recorded at the time revenues are recognized. Accordingly, the Company reduces recognized revenue for estimated future returns at the time revenue is recorded. The estimates for returns are adjusted periodically based upon changes in historical rates of returns and trend analysis.
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 — 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.
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 economic 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 years. The Company did not have a triggering event during fiscal 2018 and 2017 .
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 350 ("ASC"), "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." The new 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 2018 and fiscal 2016, the Company performed a quantitative impairment assessment for goodwill. The Company calculated the fair value of its 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 operating margins, as well as the perceived risk associated with those forecasts, along with selecting representative 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 2018, 2017 or 2016. 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 difference. 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 developed for purposes of the Company's goodwill impairment test. 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 maintains an indemnity receivable for certain tax obligations that are indemnified by Tyco International Holding S.à.r.l. ("Tyco") and that are expected to be settled directly with the taxing authorities. The Company periodically assesses the realizabilty 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.
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 2017-07 Compensation - Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost The ASU requires an entity to report the service cost component of pension cost and postretirement benefit cost as compensation expense during the employee's service period. The other components of net periodic pension benefit costs will be presented outside a subtotal of income from operations. Prior to the adoption of ASU 2017-07, pension costs were reported as cost of sales and selling, general and administrative expenses on the Company's consolidated statements of income. As a result of the early adoption of ASU 2017-07, the Company reclassified a net credit of $1,505 and $1,453 from operating income to other income, net on the consolidated statements of the years ended September 30, 2017, and September 30, 2016, respectively. 4 2018 2017-09 Compensation - Stock Compensation (Topic 718): Scope of Modification Accounting The ASU does not require an entity to apply modification accounting if the fair value, vesting conditions and classification of the awards do not change. No material impact on the consolidated financial statements. 5 2018 A summary of guidance not yet adopted are as follows: Guidance not yet adopted ASU Description of ASU Impact to Atkore Effective Date 2016-16 Income Taxes (Topic 740): Intra-Entity Transfers of Assets The ASU removes the prohibition in ASC 740 against the immediate recognition of the current and deferred income tax effects of intra-entity transfers of assets other than inventory. Under evaluation 2019 ASU 2018-02 Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income The ASU provided entities the option to reclassify tax effects stranded in accumulated other comprehensive income as a result of the Tax Cuts and Jobs Act to retained earnings. Under evaluation. 2020 2014-09 Revenue from Contracts with Customers The ASU, and subsequent amendments to the guidance issued in fiscal 2018, 2017, 2016 and 2015, provide 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 this standard in the first quarter of 2019 using the modified retrospective method. The adoption of the standard did not have a material impact on the Company’s consolidated financial statements. Based on the completion of its reviews and assessments, the Company has determined that the pattern of revenue recognition for substantially all of its existing revenue streams is substantially unchanged. The Company implemented the appropriate changes to business processes and controls to support recognition and disclosure under the new standard, including the new qualitative and quantitative disclosures that will include information on the nature, amount, timing and significant judgments impacting revenue from contracts with customers. 2019 2016-02 Leases (Topic 842) The ASU, and subsequent amendments to the guidance issued in fiscal 2018 and 2017, require 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. The Company has established an implementation team, deployed lease landscape surveys and is currently evaluating the impact of adoption of this ASU on its consolidated financial statements. 2020 2018-14 Compensation - Retirement Benefits - Defined Benefit Plans - General (Subtopic 715-20) The ASU amends ASC 715 to add, remove and clarify disclosure requirements related to defined benefit pension and other postretirement plans. Under evaluation. 2021 2018-07 Improvements to Nonemployee Share-Based Payment Accounting. ASU 2018-07 simplifies the accounting for share-based payments to nonemployees by aligning it with the accounting for share-based payments to employees, with certain exceptions. Under evaluation. 2020

Basis of Presentation and Sum_3

Basis of Presentation and Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Sep. 30, 2018
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 economic useful life Software 2 to 10 years As of September 30, 2018 and September 30, 2017 , property, plant and equipment at cost and accumulated depreciation were as follows: (in thousands) September 30, 2018 September 30, 2017 Land $ 13,295 $ 13,296 Buildings and related improvements 108,758 105,154 Machinery and equipment 262,078 248,440 Leasehold improvements 7,382 6,744 Software 30,502 15,135 Construction in progress 16,777 16,160 Property, plant and equipment 438,792 404,929 Accumulated depreciation (225,684 ) (196,310 ) Property, plant and equipment, net $ 213,108 $ 208,619
Schedule of New Accounting Pronouncements and Changes in Accounting Principles	Adopted Guidance ASU Description of ASU Impact to Atkore Note Adoption Date 2017-07 Compensation - Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost The ASU requires an entity to report the service cost component of pension cost and postretirement benefit cost as compensation expense during the employee's service period. The other components of net periodic pension benefit costs will be presented outside a subtotal of income from operations. Prior to the adoption of ASU 2017-07, pension costs were reported as cost of sales and selling, general and administrative expenses on the Company's consolidated statements of income. As a result of the early adoption of ASU 2017-07, the Company reclassified a net credit of $1,505 and $1,453 from operating income to other income, net on the consolidated statements of the years ended September 30, 2017, and September 30, 2016, respectively. 4 2018 2017-09 Compensation - Stock Compensation (Topic 718): Scope of Modification Accounting The ASU does not require an entity to apply modification accounting if the fair value, vesting conditions and classification of the awards do not change. No material impact on the consolidated financial statements. 5 2018 A summary of guidance not yet adopted are as follows: Guidance not yet adopted ASU Description of ASU Impact to Atkore Effective Date 2016-16 Income Taxes (Topic 740): Intra-Entity Transfers of Assets The ASU removes the prohibition in ASC 740 against the immediate recognition of the current and deferred income tax effects of intra-entity transfers of assets other than inventory. Under evaluation 2019 ASU 2018-02 Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income The ASU provided entities the option to reclassify tax effects stranded in accumulated other comprehensive income as a result of the Tax Cuts and Jobs Act to retained earnings. Under evaluation. 2020 2014-09 Revenue from Contracts with Customers The ASU, and subsequent amendments to the guidance issued in fiscal 2018, 2017, 2016 and 2015, provide 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 this standard in the first quarter of 2019 using the modified retrospective method. The adoption of the standard did not have a material impact on the Company’s consolidated financial statements. Based on the completion of its reviews and assessments, the Company has determined that the pattern of revenue recognition for substantially all of its existing revenue streams is substantially unchanged. The Company implemented the appropriate changes to business processes and controls to support recognition and disclosure under the new standard, including the new qualitative and quantitative disclosures that will include information on the nature, amount, timing and significant judgments impacting revenue from contracts with customers. 2019 2016-02 Leases (Topic 842) The ASU, and subsequent amendments to the guidance issued in fiscal 2018 and 2017, require 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. The Company has established an implementation team, deployed lease landscape surveys and is currently evaluating the impact of adoption of this ASU on its consolidated financial statements. 2020 2018-14 Compensation - Retirement Benefits - Defined Benefit Plans - General (Subtopic 715-20) The ASU amends ASC 715 to add, remove and clarify disclosure requirements related to defined benefit pension and other postretirement plans. Under evaluation. 2021 2018-07 Improvements to Nonemployee Share-Based Payment Accounting. ASU 2018-07 simplifies the accounting for share-based payments to nonemployees by aligning it with the accounting for share-based payments to employees, with certain exceptions. Under evaluation. 2020

Acquisitions (Tables)

Acquisitions (Tables)	12 Months Ended
	Sep. 30, 2018
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
Schedule of Fair Value of Amortizable 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
Business Acquisition, Pro Forma Information	The following table presents unaudited pro forma results of operations for the fiscal years ended September 30, 2017 and September 30, 2016 as if the Calpipe acquisition had occurred as of the first day of the fiscal 2016 period: Fiscal Year Ended (in thousands) September 30, 2017 September 30, 2016 Pro forma net sales $ 1,575,801 $ 1,589,136 Pro forma net income 91,362 62,127

Divestiture (Tables)

Divestiture (Tables)	12 Months Ended
	Sep. 30, 2018
Discontinued Operations and Disposal Groups [Abstract]
Sale Price Allocation	The cash consideration received, net assets divested and resulting gain on sale 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, 2018
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, 2018 September 30, 2017 September 30, 2016 Service cost $ — $ 2,049 $ 1,894 Interest cost 4,098 3,793 4,143 Expected return on plan assets (6,415 ) (6,601 ) (6,318 ) Amortization of actuarial loss 343 1,303 722 Net periodic (benefit)/cost $ (1,974 ) $ 544 $ 441 The weighted-average assumptions used to determine net periodic pension benefit cost during the period were as follow: September 30, 2018 September 30, 2017 September 30, 2016 Discount rate 3.7 % 3.5 % 4.2 % Expected return on plan assets 6.4 % 7.0 % 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, 2016 $ 133,152 Service cost 2,049 Interest cost 3,793 Actuarial (gain) (5,012 ) Benefits and administrative expenses paid (5,130 ) Balance as of September 30, 2017 128,852 Service cost — Interest cost 4,098 Actuarial (gain) (7,130 ) Benefits and administrative expenses paid (4,863 ) Balance as of September 30, 2018 $ 120,957 Change in plan assets: Balance as of September 30, 2016 $ 97,980 Actual return on plan assets 10,388 Employer contributions 375 Benefits and administrative expenses paid (5,130 ) 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 Funded status as of September 30, 2018 $ (16,503 )
Schedule of Amounts Recognized in Balance Sheet	(in thousands) September 30, 2018 September 30, 2017 Amounts recognized in the consolidated balance sheets consist of: Pension assets $ 572 $ — Pension liabilities (17,075 ) (25,239 ) Net amount recognized $ (16,503 ) $ (25,239 ) Amounts recognized in accumulated other comprehensive loss (before income taxes) consist of: Net actuarial loss $ (12,344 ) $ (18,103 ) Total loss recognized $ (12,344 ) $ (18,103 ) Weighted-average assumptions used to determine pension benefit obligations at year end: Discount rate 4.2 % 3.7 % 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, 2018 September 30, 2017 Accumulated benefit obligation $ 105,414 $ 128,852 Fair value of plan assets 88,339 103,613 The following table summarizes the defined benefit pension plans with projected benefit obligations in excess of plan assets: (in thousands) September 30, 2018 September 30, 2017 Projected benefit obligation $ 105,414 $ 128,852 Fair value of plan assets 88,339 103,613
Schedule of Allocation of Plan Assets	Pension plans have the following weighted-average asset allocations: Asset Category: September 30, 2018 September 30, 2017 Equity securities 59% 61% Debt securities 39% 38% Cash and cash equivalents 2% 1% Total 100% 100% The Company's asset allocations are presented in the table below: (in thousands) September 30, 2018 September 30, 2017 Level 1 Level 2 Total Level 1 Level 2 Total United States equity securities $ 32,447 $ 6,354 $ 38,801 $ — $ 36,362 $ 36,362 Non-U.S. equity securities 20,164 2,599 22,763 — 26,432 26,432 Fixed income securities 34,111 6,678 40,789 — 39,319 39,319 Cash and cash equivalents 2,101 — 2,101 1,500 — 1,500 Total $ 88,823 $ 15,631 $ 104,454 $ 1,500 $ 102,113 $ 103,613
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) 2019 $ 5,717 2020 6,011 2021 6,340 2022 6,609 2023 6,814 2024 and thereafter 36,690

Stock Incentive Plan (Tables)

Stock Incentive Plan (Tables)	12 Months Ended
	Sep. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [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, 2018 were as follows: September 30, 2018 September 30, 2017 Expected dividend yield — % — % Range of expected volatility 17.21 - 79.15 17.55 - 75.55 Risk free interest rates 1.83 - 2.54 1.35 % Expected life 1 - 3 years 3 years Fair value $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, 2018 September 30, 2017 September 30, 2016 Expected dividend yield — % — % — % Expected volatility 40 % 40 % 40 % Range of risk-free interest rates (%) 2.16-2.38 1.95 0.74 - 1.27 Range of expected option lives 1.00 to 6.00 years 6.00 years 2.09 - 6.37 years
Schedule of Stock Option Activity	Stock option activity for the period September 25, 2015 to September 30, 2018 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 25, 2015 6,746 $ 7.70 Granted 72 15.79 $ 7.63 Exercised (18 ) 7.95 $ 43 Forfeited (136 ) 8.84 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 $ 43,302 5.61 Exercisable as of September 30, 2018 1,915 $ 8.41 $ 34,697 5.04
Schedule of Nonvested Share Activity	Changes to the Company's non-vested PSU awards for the year ended September 30, 2018 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 Changes to the Company's nonvested RSU awards for the year ended September 30, 2018 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

Restructuring Charges and Ass_2

Restructuring Charges and Asset Impairments (Tables)	12 Months Ended
	Sep. 30, 2018
Restructuring and Related Activities [Abstract]
Schedule of Restructuring Reserves	Restructuring reserves are included as a component of other current liabilities . See Note 1, ''Basis of Presentation and Summary of Significant Accounting Policies.'' Electrical Raceway Mechanical Products & Solutions Other/Corporate (in thousands) Severance (a) Other (a) Severance (b) Other (b) Severance Other Total Balance as of September 25, 2015 $ 206 $ — $ 3,511 $ 620 $ 15 $ 61 $ 4,413 Charges 807 — 689 2,583 — 199 4,278 Utilization (168 ) — (4,017 ) (2,542 ) (11 ) (260 ) (6,998 ) Reversals — — (183 ) (122 ) (4 ) — (309 ) Exchange rate effects $ (4 ) $ — $ — $ — $ — $ — $ (4 ) 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 (a) Primarily related to Atkore's commitment to close certain facilities as part of its continuing effort to realign its strategic focus. The Company recorded severance restructuring charges of $240 , $297 and $778 related to termination benefits during the fiscal years ended 2018, 2017 and 2016, respectively. The Company recorded other restructuring charges to close facilities of $976 and $297 for the fiscal years ending September 30, 2018 and September 30, 2017 , respectively. (b) Primarily related to the August 6, 2015 announcement of the Company's plans to exit its Fence and Sprinkler steel pipe and tube product lines ("Fence and Sprinkler") in order to realign its long-term strategic focus. The operations associated with these product lines were wound down during the first quarter of fiscal 2016 and resulted in headcount reductions in the Company's Philadelphia, Phoenix and Harvey facilities. The Company recorded $630 of severance-related expenses for the fiscal year ended September 30, 2016 related to the headcount reductions. The Company also recorded $64 and $2,066 of facility-related and other charges related to the closure of the facilities during fiscal 2017 and 2016 respectively. The Company expects to utilize all restructuring accruals as of September 30, 2018 within the next twelve months. The net restructuring charges included as a component of selling, general and administrative expense and asset impairment charges in the Company's consolidated statements of operations were as follows: Fiscal Year Ended (in thousands) September 30, 2018 September 30, 2017 September 30, 2016 Total restructuring charges, net 1,849 1,256 3,967 Asset impairment charges — — 129

Other Income, Net (Tables)

Other Income, Net (Tables)	12 Months Ended
	Sep. 30, 2018
Other Income, Nonoperating [Abstract]
Other Income, Net	Other income, net consisted of the following: Fiscal Year Ended Fiscal Year Ended Fiscal Year Ended (in thousands) September 30, 2018 September 30, 2017 As Adjusted** September 30, 2016 As Adjusted** Gain on sale of a business $ (27,575 ) $ — $ — Gain on sale of joint venture — (5,774 ) — Undesignated foreign currency derivative instruments (121 ) 2,741 — Foreign exchange gain (loss) on intercompany loans 1,500 (2,038 ) — Debt modification costs 892 — — Pension-related benefits (1,975 ) (1,504 ) (1,452 ) Other (69 ) — — Other income, net $ (27,348 ) $ (6,575 ) $ (1,452 ) ** As adjusted due to the adoption of ASU 2017-07. See Note 1, ''Basis of Presentation and Summary of Significant Accounting Policies'' for additional information.

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Sep. 30, 2018
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, 2018 , September 30, 2017 and September 30, 2016 consisted of the following: (in thousands) September 30, 2018 September 30, 2017 September 30, 2016 Components of income before income taxes: United States $ 155,453 $ 116,580 $ 78,016 Non-U.S 10,899 9,545 8,765 Income before income taxes $ 166,352 $ 126,125 $ 86,781 Income tax expense: Current: United States: Federal $ 26,586 $ 33,127 $ 18,748 State 8,662 4,246 4,655 Non-U.S: 3,467 3,175 2,026 Current income tax expense $ 38,715 $ 40,548 $ 25,429 Deferred: United States: Federal $ (7,149 ) $ 224 $ 642 State (1,793 ) 469 1,872 Non-U.S: (66 ) 245 42 Deferred income tax (benefit) expense (9,008 ) 938 2,556 Income tax expense $ 29,707 $ 41,486 $ 27,985
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, 2018 September 30, 2017 September 30, 2016 Statutory federal tax 25 % 35 % 35 % Adjustments to reconcile to the effective income tax rate: State income taxes 3 % 3 % 5 % Nondeductible expenses — % — % 2 % Change in tax rates (3 )% — % — % Domestic manufacturing deduction (2 )% (2 )% (3 )% Indemnified uncertain tax benefits (2 )% — % (5 )% Stock-based compensation (3 )% (3 )% — % Other — % — % (2 )% Effective income tax rate 18 % 33 % 32 %
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, 2018 September 30, 2017 Deferred tax assets: Accrued liabilities and reserves $ 32,099 $ 48,619 Tax loss and credit carryforwards 12,435 15,340 Postretirement benefits 4,317 9,863 Inventory 11,220 6,482 Other 218 877 $ 60,289 $ 81,181 Deferred tax liabilities: Property, plant and equipment $ (13,739 ) $ (13,770 ) Intangible assets (48,375 ) (65,072 ) Loss on investment (5,321 ) (6,085 ) Other (1,775 ) (2,549 ) $ (69,210 ) $ (87,476 ) Net deferred tax liability before valuation allowance (8,921 ) (6,295 ) Valuation allowance (7,427 ) (9,512 ) Net deferred tax liability $ (16,348 ) $ (15,807 )
Schedule of Reconciliation of Unrecognized Tax Benefits, Excluding Interest and Penalties	A reconciliation of the beginning and ending amount of unrecognized tax benefit, excluding interest and penalties, is as follows: (in thousands) For the period from September 30, 2016 to September 30, 2018 Balance as of September 25, 2015 $ 8,101 Additions based on tax positions related to prior years 62 Settlements (4,360 ) 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
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 2014, 2016, and 2017

Earnings Per Share (Tables)

Earnings Per Share (Tables)	12 Months Ended
	Sep. 30, 2018
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, 2018 September 30, 2017 September 30, 2016 Numerator: Net income $ 136,645 $ 84,639 $ 58,796 Less: Undistributed earnings allocated to participating securities 2,456 241 32 Net income available to common shareholders $ 134,189 $ 84,398 $ 58,764 Denominator: Basic weighted average common shares outstanding 51,791 63,420 62,486 Effect of dilutive securities: Non-participating employee stock options (1) 2,298 3,134 300 Diluted weighted average common shares outstanding 54,089 66,554 62,786 Basic earnings per share $ 2.59 $ 1.33 $ 0.94 Diluted earnings per share $ 2.48 $ 1.27 $ 0.94 (1) Stock options to purchase approximately 0.3 million , 0.2 million , and 0.1 million shares of common stock were outstanding during the years ended September 30, 2018 , September 30, 2017, and September 30, 2016, 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, 2018
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, 2016 $ (17,192 ) $ (8,758 ) $ (25,950 ) Other comprehensive loss before reclassifications 5,941 1,221 7,162 Amounts reclassified from accumulated other comprehensive loss 806 — 806 Net current period other comprehensive loss 6,747 1,221 7,968 Balance as of September 30, 2017 $ (10,445 ) $ (7,537 ) $ (17,982 ) Other comprehensive income 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 )
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, 2018 September 30, 2017 September 30, 2016 Amortization of defined benefit pension items: Amortization of net loss (included within other income, net) $ 343 $ 1,303 $ 722 Tax expense (14 ) (497 ) (276 ) Net reclassifications for the period $ 329 $ 806 $ 446 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 $101 .

Inventories, Net (Tables)

Inventories, Net (Tables)	12 Months Ended
	Sep. 30, 2018
Inventory Disclosure [Abstract]
Schedule of Inventory, Net	Approximately 80% and 75% of the Company's inventories are valued at the lower of LIFO cost or market at September 30, 2018 and September 30, 2017 , respectively. (in thousands) September 30, 2018 September 30, 2017 Purchased materials and manufactured parts, net $ 58,572 $ 49,168 Work in process, net 21,769 17,598 Finished goods, net 141,412 133,237 Inventories, net $ 221,753 $ 200,003

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	12 Months Ended
	Sep. 30, 2018
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 economic useful life Software 2 to 10 years As of September 30, 2018 and September 30, 2017 , property, plant and equipment at cost and accumulated depreciation were as follows: (in thousands) September 30, 2018 September 30, 2017 Land $ 13,295 $ 13,296 Buildings and related improvements 108,758 105,154 Machinery and equipment 262,078 248,440 Leasehold improvements 7,382 6,744 Software 30,502 15,135 Construction in progress 16,777 16,160 Property, plant and equipment 438,792 404,929 Accumulated depreciation (225,684 ) (196,310 ) Property, plant and equipment, net $ 213,108 $ 208,619

Goodwill and Intangible Assets

Goodwill and Intangible Assets (Tables)	12 Months Ended
	Sep. 30, 2018
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, 2016 $ 76,640 $ 39,189 $ 115,829 Goodwill acquired during year 31,477 — $ 31,477 Exchange rate effects 410 — $ 410 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
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, 2018 September 30, 2017 (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 $ 330,295 $ (141,401 ) $ 188,894 $ 350,129 $ (118,273 ) $ 231,856 Other 8 16,003 (5,861 ) 10,142 27,819 (9,266 ) 18,553 Total 346,298 (147,262 ) 199,036 377,948 (127,539 ) 250,409 Indefinite-lived Intangible Assets: Trade names 92,880 — 92,880 93,880 — 93,880 Total $ 439,178 $ (147,262 ) $ 291,916 $ 471,828 $ (127,539 ) $ 344,289
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, 2018 September 30, 2017 (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 $ 330,295 $ (141,401 ) $ 188,894 $ 350,129 $ (118,273 ) $ 231,856 Other 8 16,003 (5,861 ) 10,142 27,819 (9,266 ) 18,553 Total 346,298 (147,262 ) 199,036 377,948 (127,539 ) 250,409 Indefinite-lived Intangible Assets: Trade names 92,880 — 92,880 93,880 — 93,880 Total $ 439,178 $ (147,262 ) $ 291,916 $ 471,828 $ (127,539 ) $ 344,289
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) : 2019 $ 31,122 2020 29,401 2021 29,267 2022 27,981 2023 27,860 2024 and thereafter 53,405

Debt (Tables)

Debt (Tables)	12 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Schedule of Debt	Debt as of September 30, 2018 and September 30, 2017 was as follows: (in thousands) September 30, 2018 September 30, 2017 First Lien Term Loan Facility due December 22, 2023 $ 912,162 $ 495,134 ABL Credit Facility — 85,000 Deferred financing costs (8,194 ) (4,496 ) Other 279 440 Total debt $ 904,247 $ 576,078 Less: Current portion (a) 26,561 4,215 Long-term debt $ 877,686 $ 571,863 (a) Of the amount due within 2019 for the First Lien Term Loan Facility, $18,936 relates to an accelerated repayment of principal calculated by a formula based on 2018 excess cash flows and a leverage ratio as defined within the Term Loan Agreement. No such accelerated payments are indicated for any of the periods beyond 2019.
Contractual Obligation, Fiscal Year Maturity Schedule	As of September 30, 2018 , future contractual maturities of long-term debt are as follows (in thousands) : 2019 (a) $ 28,136 2020 9,200 2021 9,200 2022 9,200 2023 9,200 2024 and thereafter $ 848,164

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Sep. 30, 2018
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, 2018 and September 30, 2017 in accordance with the fair value hierarchy: September 30, 2018 September 30, 2017 (in thousands) Level 1 Level 2 Level 3 Level 1 Level 2 Level 3 Assets Cash equivalents $ 28,175 $ — $ — $ 571 $ — $ — Liabilities Forward currency contracts — $ 1,857 — — $ 2,936 —
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, 2018 September 30, 2017 (in thousands) Carrying Value Fair Value Carrying Value Fair Value First Lien Term Loan Facility due December 22, 2023 $ 913,100 $ 916,113 $ 496,250 $ 498,979 Total debt $ 913,100 $ 916,113 $ 496,250 $ 498,979

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Sep. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Schedule of Minimum Future Operating Lease Payments	At September 30, 2018 , minimum future operating lease payments for leases with terms in excess of one year are presented in the table below as follows (in thousands): 2019 $ 11,069 2020 8,226 2021 6,580 2022 4,886 2023 4,254 2024 and thereafter 9,988 Total $ 45,003

Segment Information (Tables)

Segment Information (Tables)	12 Months Ended
	Sep. 30, 2018
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, 2018 September 30, 2017 September 30, 2016 Operating segment Adjusted EBITDA Electrical Raceway $ 255,260 $ 189,351 $ 181,939 MP&S 51,339 63,687 81,199 Total $ 306,599 $ 253,038 $ 263,138 Unallocated expenses (a) (35,050 ) (25,430 ) (28,136 ) Depreciation and amortization (66,890 ) (54,727 ) (55,017 ) Interest expense, net (40,694 ) (26,598 ) (41,798 ) Gain (loss) on extinguishment of debt — (9,805 ) 1,661 Restructuring & impairments (1,849 ) (1,256 ) (4,096 ) Net periodic pension benefit cost — — (441 ) Stock-based compensation (14,664 ) (12,788 ) (21,127 ) Special products claims impact — — (850 ) Certain legal matters 4,833 (7,551 ) (1,382 ) Consulting fees — — (15,425 ) Transaction costs (9,314 ) (4,779 ) (7,832 ) Gain on sale of a business 27,575 — — Gain on sale of joint venture — 5,774 — Other (b) (4,194 ) 10,247 (1,103 ) Impact of Fence and Sprinkler exit — — (811 ) Income before income taxes $ 166,352 $ 126,125 $ 86,781 (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. 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, 2018 September 30, 2017 September 30, 2016 (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 $ 1,365,067 $ 1,544 $ 255,260 $ 1,093,500 $ 1,283 $ 189,351 $ 1,066,711 $ 1,919 $ 181,939 MP&S 470,072 81 $ 51,339 410,434 98 $ 63,687 456,673 148 $ 81,199 Eliminations — (1,625 ) — (1,381 ) — (2,067 ) Consolidated operations $ 1,835,139 $ — $ 1,503,934 $ — $ 1,523,384 $ — Capital Expenditures Total Assets (in thousands) September 30, 2018 September 30, 2017 September 30, 2016 September 30, 2018 September 30, 2017 September 30, 2016 Electrical Raceway $ 16,389 $ 13,037 $ 9,161 $ 751,024 $ 757,775 $ 566,250 MP&S 14,267 8,212 6,130 291,164 306,229 343,002 Unallocated 7,845 3,873 1,539 281,872 151,088 255,316 Consolidated operations $ 38,501 $ 25,122 $ 16,830 $ 1,324,060 $ 1,215,092 $ 1,164,568
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, 2018 September 30, 2017 September 30, 2016 September 30, 2018 September 30, 2017 September 30, 2016 United States $ 201,101 $ 202,823 $ 204,640 $ 1,651,636 $ 1,367,907 $ 1,395,750 Other Americas 138 164 175 43,013 37,908 40,573 Europe 11,090 9,306 1,295 90,915 55,181 40,246 Asia-Pacific 2,386 3,378 3,826 49,575 42,938 46,815 Total $ 214,715 $ 215,671 $ 209,936 $ 1,835,139 $ 1,503,934 $ 1,523,384
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, 2018 September 30, 2017 September 30, 2016 Metal Electrical Conduit and Fittings $ 517,935 $ 349,239 $ 331,526 Armored Cable and Fittings 336,388 323,070 318,279 PVC Electrical Conduit & Fittings 311,811 265,389 258,954 Other raceway products 198,933 155,802 157,952 Electrical Raceway 1,365,067 1,093,500 1,066,711 Mechanical Pipe 253,381 211,245 249,473 Other MP&S products 216,691 199,189 199,384 Impact of Fence and Sprinkler — — 7,816 MP&S 470,072 410,434 456,673 Net sales $ 1,835,139 $ 1,503,934 $ 1,523,384

Quarterly Operating Results (Ta

Quarterly Operating Results (Tables)	12 Months Ended
	Sep. 30, 2018
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, 2018 and September 30, 2017 . 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 29, 2017 March 30, 2018 June 29, 2018 September 30, 2018 December 30, 2016 As Adjusted ** (1) March 31, 2017 As Adjusted ** (2) June 30, 2017 As Adjusted ** September 30, 2017 As Adjusted ** Net sales $ 414,558 $ 445,000 $ 498,014 $ 477,567 $ 337,591 $ 372,791 $ 397,745 $ 395,807 Gross profit 96,867 109,157 120,329 111,731 91,665 87,609 92,485 89,513 Net income 27,189 42,558 34,199 32,699 17,382 18,935 27,465 20,857 Net income per share Basic (6) $ 0.43 $ 0.83 $ 0.73 $ 0.69 $ 0.28 $ 0.30 $ 0.43 $ 0.33 Diluted (6) 0.41 0.79 0.70 0.66 0.26 0.28 0.41 0.31 ** As adjusted due to the adoption of ASU 2017-07. See Note 1, ''Basis of Presentation and Summary of Significant Accounting Policies'' for additional information. (1) Includes a $9,805 loss on the extinguishment of the Initial First Lien Term Loan Facility and the Second Lien Term Loan Facility. See Note 14, "Debt." (2) Includes $5,774 pre-tax gain on sale of Abahsain-Cope Saudi Arabia Ltd joint venture and $7,501 pre-tax expense related to the Antidumping Duty Order for Malleable Iron Pipe Fittings. See Note 16, "Commitments and contingencies" for additional information (3) Includes a $4,758 benefit due to the re-measurement of deferred tax liabilities as a result of the TCJA. See Note 8, "Income taxes" for additional information. (4) 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 3, "Divestitures" for additional information. (5) Includes $7,171 pre- tax reversal of expense related to the Antidumping Duty Order for Malleable Iron Pipe Fittings. See Note 16, Commitments and contingencies" for additional information. (6) 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	Mar. 06, 2014	Dec. 22, 2010
Debt Instrument [Line Items]
Dividend rate, percentage (%)		12.00%
Payment to redeem shares	$ 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

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, 2018	Sep. 30, 2017	Aug. 31, 2017
Related Party Transaction [Line Items]
Common stock, par value (in dollars per share)		$ 0.01	$ 0.01
Repurchase of common stock		$ 411,775,000	$ 13,938,000
Stock repurchase program, authorized amount				$ 75,000,000
Remaining authorized repurchase amount			$ 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, 2018
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, 2018
Finite-Lived Intangible Assets [Line Items]
Useful life (years)
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	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Assets Sold under Agreements to Repurchase [Line Items]
Other nonoperating income (expense)	$ 27,348	$ 6,575	$ 1,452
Accounting Standards Update 2017-07
Assets Sold under Agreements to Repurchase [Line Items]
Other nonoperating income (expense)		$ 1,505	$ 1,453

Acquisitions - Narrative (Detai

Acquisitions - Narrative (Details) $ in Thousands	Oct. 01, 2018USD ($)	Jan. 08, 2018USD ($)	May 18, 2017USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)	Sep. 27, 2018Countires	Sep. 01, 2017Countires
Business Acquisition [Line Items]
Purchase price adjustments				$ 24,527
Intangibles, purchase accounting adjustments				18,800
Property, plant, and equipment, purchase accounting adjustments				2,500
Deferred tax liabilities, purchase accounting adjustments				$ 8,200
Amortization period for tax-deductible goodwill				15 years
Cash consideration			$ 197,804		$ 0
Communications Integrators, Inc
Business Acquisition [Line Items]
Cash consideration, net of cash acquired		$ 3,997
Calpipe Industries
Business Acquisition [Line Items]
Acquisition-related expenses				$ 558	1,676
Cash consideration			110,155
Other Acquisitions
Business Acquisition [Line Items]
Cash consideration			$ 87,649
Flexicon Limited
Business Acquisition [Line Items]
Number of countries in which entity operates | Countires								55
Vergokan
Business Acquisition [Line Items]
Number of countries in which entity operates | Countires							45
Fair Value Adjustment to Inventory
Business Acquisition [Line Items]
Pro forma adjustment to reflect results of operations					$ 2,200	$ 2,200
Subsequent Event | Vergokan
Business Acquisition [Line Items]
Purchase price for outstanding stock	$ 58,728

Acquisitions - Net Assets Acqui

Acquisitions - Net Assets Acquired and Liabilites Assumed Fiscal 2017 Transaction (Details) - USD ($) $ in Thousands	May 18, 2017	Sep. 30, 2017
Fair value of consideration transferred:
Cash consideration	$ 197,804	$ 0
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
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	87,649
Working capital adjustment	0
Purchase price payable	0
Settlement of pre-existing relationship	0
Total consideration transferred	87,649
Fair value of assets acquired and liabilities assumed:
Cash	8,830
Accounts receivable	7,589
Inventories	7,221
Intangible assets	40,100
Fixed assets	11,242
Accounts payable	(1,550)
Other	(11,558)
Net assets acquired	61,874
Excess purchase price attributed to goodwill acquired	$ 25,775

Acquisitions - Fair Value of Am

Acquisitions - Fair Value of Amortizable Intangible Assets Fiscal 2017 Transaction (Details) $ in Thousands	May 18, 2017USD ($)
Business Acquisition [Line Items]
Intangible assets	$ 94,960
Other Acquisitions
Business Acquisition [Line Items]
Weighted Average Useful Life (Years)	9 years 10 months 25 days
Intangible assets	$ 40,100
Calpipe Industries
Business Acquisition [Line Items]
Weighted Average Useful Life (Years)	9 years 10 months 10 days
Intangible assets	$ 54,860
Customer relationships | Other Acquisitions
Business Acquisition [Line Items]
Fair Value	$ 37,341
Weighted Average Useful Life (Years)	10 years
Customer relationships | Calpipe Industries
Business Acquisition [Line Items]
Fair Value	$ 50,680
Weighted Average Useful Life (Years)	9 years 11 months 19 days
Other | Other Acquisitions
Business Acquisition [Line Items]
Fair Value	$ 2,759
Weighted Average Useful Life (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 5 months 16 days

Acquisitions Acquisitions - Pro

Acquisitions Acquisitions - Pro Forma Results (Details) - Calpipe Industries - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Business Acquisition [Line Items]
Pro forma net sales	$ 1,575,801	$ 1,589,136
Pro forma net income	$ 91,362	$ 62,127

Divestiture (Details)

Divestiture (Details) - USD ($) $ in Thousands	Mar. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash consideration		$ 0	$ 3,024	$ 2,400
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 - Net P

Postretirement Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Net periodic (benefit)/cost	$ 0	$ 0	$ 441
Weighted-average assumptions used to determine net periodic pension cost during the period:
Discount rate	3.70%	3.50%	4.20%
Expected return on plan assets	6.40%	7.00%	7.00%
Pension Plan
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 0	$ 2,049	$ 1,894
Interest cost	4,098	3,793	4,143
Expected return on plan assets	(6,415)	(6,601)	(6,318)
Amortization of actuarial loss	343	1,303	722
Net periodic (benefit)/cost	$ (1,974)	$ 544	$ 441

Postretirement Benefits - Narra

Postretirement Benefits - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plans expense	$ 3,600	$ 3,000	$ 2,817
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	1,003	375
Estimated employer contributions in next fiscal year	$ 1,077
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	$ 5,934	$ 6,250

Postretirement Benefits - Chang

Postretirement Benefits - Change in Benefit Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Change in benefit obligations:
Actuarial (gain)	$ (101)
Amounts recognized in the consolidated balance sheets consist of:
Pension liabilities	(17,075)	$ (25,239)
Pension Plan
Change in benefit obligations:
Beginning balance, benefit obligations	128,852	133,152
Service cost	0	2,049	$ 1,894
Interest cost	4,098	3,793	4,143
Actuarial (gain)	(7,130)	(5,012)
Benefits and administrative expenses paid	(4,863)	(5,130)
Ending balance, benefit obligations	120,957	128,852	133,152
Change in plan assets:
Beginning balance, fair value of plan assets	103,613	97,980
Actual return on plan assets	4,701	10,388
Employer contributions	1,003	375
Benefits and administrative expenses paid	(4,863)	(5,130)
Ending balance, fair value of plan assets	104,454	103,613	$ 97,980
Amounts recognized in the consolidated balance sheets consist of:
Pension assets	572	0
Pension liabilities	(17,075)	(25,239)
Net amount recognized	(16,503)	(25,239)
Amounts recognized in accumulated other comprehensive loss (before income taxes) consist of:
Net actuarial loss	(12,344)	(18,103)
Total loss recognized	$ (12,344)	$ (18,103)
Weighted-average assumptions used to determine pension benefit obligations at year end:
Discount rate	4.20%	3.70%

Postretirement Benefits - Benef

Postretirement Benefits - Benefit Obligation In Excess Of Plan Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan, Plan with Benefit Obligation in Excess of Plan Assets [Abstract]
Accumulated benefit obligation	$ 105,414	$ 128,852
Fair value of plan assets	88,339	103,613
Defined Benefit Plan, Pension Plan with Accumulated Benefit Obligation in Excess of Plan Assets [Abstract]
Projected benefit obligation	105,414	128,852
Fair value of plan assets	$ 88,339	$ 103,613

Postretirement Benefits - Weigh

Postretirement Benefits - Weighted-Average Asset Allocations (Details) - Pension Plan	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Plan asset allocations	100.00%	100.00%
Equity securities
Defined Benefit Plan Disclosure [Line Items]
Plan asset allocations	59.00%	61.00%
Debt securities
Defined Benefit Plan Disclosure [Line Items]
Plan asset allocations	39.00%	38.00%
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Plan asset allocations	2.00%	1.00%

Postretirement Benefits - Asset

Postretirement Benefits - Asset Allocations by Fair Value Hierarchy (Details) - Pension Plan - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 104,454	$ 103,613	$ 97,980
Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	40,789	39,319
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,101	1,500
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	88,823	1,500
Level 1 | Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	34,111	0
Level 1 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,101	1,500
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15,631	102,113
Level 2 | Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,678	39,319
Level 2 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States equity securities | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	38,801	36,362
United States equity securities | Level 1 | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	32,447	0
United States equity securities | Level 2 | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,354	36,362
Non-U.S. equity securities | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	22,763	26,432
Non-U.S. equity securities | Level 1 | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	20,164	0
Non-U.S. equity securities | Level 2 | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 2,599	$ 26,432

Postretirement Benefits - Futur

Postretirement Benefits - Future Expected Benefit Payments (Details) $ in Thousands	Sep. 30, 2018USD ($)
Retirement Benefits [Abstract]
2,019	$ 5,717
2,020	6,011
2,021	6,340
2,022	6,609
2,023	6,814
2024 and thereafter	$ 36,690

Stock Incentive Plan - Narrativ

Stock Incentive Plan - Narrative (Details) - USD ($) $ in Thousands, shares in Millions	Jun. 10, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense		$ 14,664	$ 12,788	$ 21,127
Employee service share-based compensation, tax benefit from compensation expense		$ 3,681	$ 4,731	$ 7,704
Omnibus Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance (shares)	3.8
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average period		1 year 10 months 14 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 10 months 25 days
Additional expense		$ 500

Stock Incentive Plan - Stock Op

Stock Incentive Plan - Stock Options Narrative (Details) $ / shares in Units, $ in Thousands	Jul. 27, 2016USD ($)employee	Sep. 30, 2018USD ($)$ / shares	Sep. 30, 2017USD ($)$ / shares	Sep. 30, 2016USD ($)$ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Reclassified from non-current liabilities to additional-paid-in-capital				$ 43,870
Stock-based compensation		$ 20,110	$ 12,168	52
Cash paid to settle options exercised		$ 0	$ 0	$ 43
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant date fair value of options granted | $ / shares		$ 8.33	$ 8.84	$ 7.63
Exercised, aggregate intrinsic value		$ 42,512	$ 25,757	$ 43
Unrecognized compensation expense		$ 2,911
Weighted-average period		1 year 4 months 9 days
Fair value of shares vested		$ 9,757	10,696	12,507
Tax benefit for tax deductions from stock options exercised		$ 11,478	$ 9,530	$ 19
Omnibus Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of employees | employee	63
Omnibus Incentive Plan | Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period		3 years
Reclassified from non-current liabilities to additional-paid-in-capital	$ 43,870
Additional expense	$ 2,400
Stock Incentive Plan | Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period		5 years

Stock Incentive Plan - Stock _2

Stock Incentive Plan - Stock Options Assumptions (Details) - Stock Options	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	0.00%	0.00%	0.00%
Expected volatility	40.00%	40.00%	40.00%
Range of risk-free interest rates (%)		195.00%
Range of risk free interest rates, minimum	2.16%		0.74%
Range of risk free interest rates, maximum	2.38%		1.27%
Range of expected option lives		6 years
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Range of expected option lives	1 year		2 years 1 month 2 days
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Range of expected option lives	6 years		6 years 4 months 13 days

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, 2018	Sep. 30, 2017	Sep. 30, 2016
Shares (in thousands)
Outstanding, beginning balance (shares)	5,158	6,664	6,746
Granted (shares)	185	171	72
Exercised (shares)	(2,728)	(1,629)	(18)
Forfeited (shares)	(10)	(48)	(136)
Outstanding, ending balance (shares)	2,605	5,158	6,664
Vested (shares)	1,915
Weighted-Average Exercise Price
Outstanding, beginning balance (in dollars per share)	$ 8.30	$ 7.76	$ 7.70
Granted (in dollars per share)	20.01	21.45	15.79
Exercised (in dollars per share)	7.55	7.47	7.95
Forfeited (in dollars per share)	8.28	9.02	8.84
Outstanding, ending balance (in dollars per share)	9.91	8.30	7.76
Vested (in dollars per share)	8.41
Stock Options, Additional Disclosures
Weighted average grant date fair value of options granted	$ 8.33	$ 8.84	$ 7.63
Exercised, aggregate intrinsic value	$ 42,512	$ 25,757	$ 43
Outstanding, aggregate intrinsic value	43,302
Vested, aggregate intrinsic value	$ 34,697
Outstanding, weighted average remaining contractual term (in years)	5 years 7 months 10 days
Vested, weighted average remaining contractual term (in years)	5 years 15 days

Stock Incentive Plan - Restrict

Stock Incentive Plan - Restricted Stock Narrative (Details) - Restricted Stock Units (RSUs) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued (shares)	334	443
Grant date fair value (in dollars per share)	$ 20.86	$ 19.68
Unrecognized compensation expense	$ 7,550
Weighted-average period	1 year 10 months 25 days
Fair value of shares vested in period	$ 1,969	$ 0	$ 300

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, 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)	435	0
Nonvested, granted (shares)	334	443
Nonvested, vested (shares)	(90)
Nonvested, forfeited (shares)	(29)	(8)
Nonvested, ending balance (shares)	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)	$ 19.65	$ 0
Nonvested, granted (in dollars per share)	20.86	19.68
Nonvested, vested (in dollars per share)	21.95
Nonvested, forfeited (in dollars per share)	20.57	21.45
Nonvested, beginning of year (in dollars per share)	$ 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, 2018	Sep. 30, 2017	Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation		$ 20,110	$ 12,168	$ 52
Closing stock price (in dollars per share)		$ 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		$ 4,472
Share-based compensation arrangement by share-based payment award, equity instruments other than options, vested in period		0	0	0

Stock Incentive Plan - Perfor_2

Stock Incentive Plan - Performance Share Assumptions (Details) - Performance Shares - $ / shares	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	0.00%	0.00%
Expected volatility, minimum	1721.00%	1755.00%
Expected volatility, maximum	7915.00%	7555.00%
Range of risk-free interest rates (%)		1.35%
Expected life		3 years
Fair value		$ 29.53
Range of risk-free interest rates, minimum (%)	1.83%
Range of risk-free interest rates, maximum (%)	2.54%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life	1 year
Fair value	$ 13.90
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life	3 years
Fair value	$ 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, 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)	181	0
Shares issued (shares)	215	184
Nonvested, forfeited (shares)	(89)	(3)
Nonvested, ending balance (shares)	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)	$ 23.87	$ 0
Grant date fair value (in dollars per share)	22.01	23.87
Nonvested, forfeited (in dollars per share)	28.53	23.87
Nonvested, beginning of year (in dollars per share)	$ 21.22	$ 23.87

Restructuring Charges and Ass_3

Restructuring Charges and Asset Impairments - Restructuring Reserves (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Restructuring Reserve [Roll Forward]
Beginning balance	$ 737	$ 1,380	$ 4,413
Charges	2,040	1,522	4,278
Utilization	(2,043)	(1,919)	(6,998)
Reversals	(191)	(266)	(309)
Exchange rate effects	8	20	(4)
Ending balance	551	737	1,380
Severance costs			630
Business exit costs		64	2,066
Other/Corporate | Severance
Restructuring Reserve [Roll Forward]
Beginning balance	0	0	15
Charges	98	71	0
Utilization	(98)	(71)	(11)
Reversals	0	0	(4)
Exchange rate effects	0	0	0
Ending balance	0	0	0
Other/Corporate | Other
Restructuring Reserve [Roll Forward]
Beginning balance	0	0	61
Charges	0	0	199
Utilization	0	0	(260)
Reversals	0	0	0
Exchange rate effects	0	0	0
Ending balance	0	0	0
Electrical Raceway | Operating Segments | Severance
Restructuring Reserve [Roll Forward]
Beginning balance	449	841	206
Charges	536	527	807
Utilization	(787)	(917)	(168)
Reversals	0	0	0
Exchange rate effects	14	(2)	(4)
Ending balance	212	449	841
Electrical Raceway | Operating Segments | Other
Restructuring Reserve [Roll Forward]
Beginning balance	0	0	0
Charges	1,130	439	0
Utilization	(820)	(209)	0
Reversals	0	(230)	0
Exchange rate effects	0	0	0
Ending balance	310	0	0
Mechanical Products & Solutions | Operating Segments | Severance
Restructuring Reserve [Roll Forward]
Beginning balance	278	0	3,511
Charges	97	422	689
Utilization	(178)	(166)	(4,017)
Reversals	(191)	0	(183)
Exchange rate effects	(6)	22	0
Ending balance	0	278	0
Mechanical Products & Solutions | Operating Segments | Other
Restructuring Reserve [Roll Forward]
Beginning balance	10	539	620
Charges	179	63	2,583
Utilization	(160)	(556)	(2,542)
Reversals	0	(36)	(122)
Exchange rate effects	0	0	0
Ending balance	$ 29	$ 10	$ 539

Restructuring Charges and Ass_4

Restructuring Charges and Asset Impairments - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Sep. 25, 2015
Restructuring Cost and Reserve [Line Items]
Charges	$ 2,040	$ 1,522	$ 4,278
Restructuring reserve	551	737	1,380	$ 4,413
Asset impairment charges	0	0	129
One-time Termination Benefits
Restructuring Cost and Reserve [Line Items]
Charges	240	297	778
Facility Closing [Member]
Restructuring Cost and Reserve [Line Items]
Charges	976	297
Prepaid Shop Supplies
Restructuring Cost and Reserve [Line Items]
Asset impairment charges	0	0	129
Operating Segments | Electrical Raceway | Severance
Restructuring Cost and Reserve [Line Items]
Charges	536	527	807
Restructuring reserve	$ 212	$ 449	$ 841	$ 206

Restructuring Charges and Ass_5

Restructuring Charges and Asset Impairments - Schedule of Restructuring Charges, Net (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Restructuring Cost and Reserve [Line Items]
Total restructuring charges, net	$ 1,849	$ 1,256	$ 3,967
Asset impairment charges	0	0	129
Prepaid Shop Supplies
Restructuring Cost and Reserve [Line Items]
Asset impairment charges	$ 0	$ 0	$ 129

Other Income, Net - Schedule of

Other Income, Net - Schedule of Other Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Other Income, Nonoperating [Abstract]
Gain on sale of a business	$ (27,575)	$ 0	$ 0
Gain on sale of joint venture	0	(5,774)	0
Undesignated foreign currency derivative instruments	(121)	2,741	0
Foreign exchange gain (loss) on intercompany loans	1,500	(2,038)	0
Debt modification costs	892	0	0
Pension-related benefits	(1,975)	(1,504)	(1,452)
Other	(69)	0	0
Other income, net	$ 27,348	$ 6,575	$ 1,452

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, 2018	Sep. 30, 2017	Sep. 30, 2016
Components of income before income taxes:
United States	$ 155,453	$ 116,580	$ 78,016
Non-U.S	10,899	9,545	8,765
Income before income taxes	166,352	126,125	86,781
United States:
Federal	26,586	33,127	18,748
State	8,662	4,246	4,655
Non-U.S:	3,467	3,175	2,026
Current income tax expense	38,715	40,548	25,429
United States:
Federal	(7,149)	224	642
State	(1,793)	469	1,872
Non-U.S:	(66)	245	42
Deferred income tax (benefit) expense	(9,008)	938	2,556
Income tax expense	$ 29,707	$ 41,486	$ 27,985

Income Taxes - Differences Betw

Income Taxes - Differences Between Federal Income Tax Rate and Effective Income Tax Rate (Details)	Dec. 22, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Income Tax Disclosure [Abstract]
Statutory federal tax	21.00%	25.00%	35.00%	35.00%
Adjustments to reconcile to the effective income tax rate:
State income taxes		3.00%	3.00%	5.00%
Nondeductible expenses		0.00%	0.00%	2.00%
Change in tax rates		(3.00%)	(0.00%)	(0.00%)
Domestic manufacturing deduction		(2.00%)	(2.00%)	(3.00%)
Indemnified uncertain tax benefits		(2.00%)	(0.00%)	(5.00%)
Stock-based compensation		(3.00%)	(3.00%)	(0.00%)
Other		0.00%	0.00%	(2.00%)
Effective income tax rate		18.00%	33.00%	32.00%

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($) $ in Thousands	Dec. 22, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Sep. 25, 2015
Operating Loss Carryforwards [Line Items]
Blended federal corporate tax rate (%)	24.50%
Statutory federal tax	21.00%	25.00%	35.00%	35.00%
Effective income tax rate reconciliation, effect of Tax Cuts and Jobs Act of 2017, amount	$ 4,758	$ 708
Tax benefit from release of indemnified uncertain tax positions		5,466	$ 3,589	$ 4,332
Effective income tax reconciliation, excess tax benefit stock option		4,091
Release of indemnification tax positions		3,977
Tax benefit for manufacturing deduction		3,309	2,761	2,805
State income tax expense		$ 5,675	3,459	4,625
Non-deductible transaction costs				1,685
Operating loss carryforwards expiration term (year)		5 years
Cumulative loss position (year)		3 years
Unrecognized tax benefits		$ 1,364	3,578	3,803	$ 8,101
Accrued interest and penalties		78	3,193
Settlements		(3,107)	(288)	(4,360)
Undistributed income		39,978
Other Deferred Tax Assets
Operating Loss Carryforwards [Line Items]
Reestablished valuation allowance deferred tax assets		600
Tyco
Operating Loss Carryforwards [Line Items]
Settlements		(3,107)	(288)	(4,360)
Increase from settlments		893
Increase (decrease) in uncertain tax positions related to accrued penalties and interest		3,139	$ 158	$ 596
Tyco | Income Tax Liabilities
Operating Loss Carryforwards [Line Items]
Offsetting receivable		122
State
Operating Loss Carryforwards [Line Items]
Operating loss carrying forwards		43,312
Non-U.S.
Operating Loss Carryforwards [Line Items]
Operating loss carrying forwards		$ 38,434

Income Taxes - Components of Ne

Income Taxes - Components of Net Deferred Income Tax Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Deferred tax assets:
Accrued liabilities and reserves	$ 32,099	$ 48,619
Tax loss and credit carryforwards	12,435	15,340
Postretirement benefits	4,317	9,863
Inventory	11,220	6,482
Other	218	877
Deferred tax assets	60,289	81,181
Deferred tax liabilities:
Property, plant and equipment	(13,739)	(13,770)
Intangible assets	(48,375)	(65,072)
Loss on investment	(5,321)	(6,085)
Other	(1,775)	(2,549)
Deferred tax liabilities	(69,210)	(87,476)
Net deferred tax liability before valuation allowance	8,921	6,295
Valuation allowance	(7,427)	(9,512)
Net deferred tax liability	$ (16,348)	$ (15,807)

Income Taxes - Unrecognized Tax

Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance	$ 3,578	$ 3,803	$ 8,101
Additions based on tax positions related to prior years	893	63	62
Settlements	(3,107)	(288)	(4,360)
Ending balance	$ 1,364	$ 3,578	$ 3,803

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, 2018	Jun. 29, 2018	Mar. 30, 2018	Dec. 29, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 30, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Numerator:
Net income	$ 32,699	$ 34,199	$ 42,558	$ 27,189	$ 20,857	$ 27,465	$ 18,935	$ 17,382	$ 136,645	$ 84,639	$ 58,796
Less: Undistributed earnings allocated to participating securities									2,456	241	32
Net income available to common shareholders									$ 134,189	$ 84,398	$ 58,764
Denominator:
Basic weighted average common shares outstanding (shares)									51,791	63,420	62,486
Effect of dilutive securities: non-participating employee stock options (shares)									2,298	3,134	300
Diluted weighted average common shares outstanding (shares)									54,089	66,554	62,786
Basic (in dollars per share)	$ 0.69	$ 0.73	$ 0.83	$ 0.43	$ 0.33	$ 0.43	$ 0.30	$ 0.28	$ 2.59	$ 1.33	$ 0.94
Diluted (in dollars per share)	$ 0.66	$ 0.70	$ 0.79	$ 0.41	$ 0.31	$ 0.41	$ 0.28	$ 0.26	$ 2.48	$ 1.27	$ 0.94
Common Stock
Denominator:
Antidilutive securities excluded from computation of earnings per share, amount									300	200	100

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, 2018	Sep. 30, 2017	Sep. 30, 2016
Tax expense
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ (10,445)	$ (17,192)
Other comprehensive income (loss), before reclassifications, net of tax	4,068	5,941
Amounts reclassified from accumulated other comprehensive loss	329	806	$ 446
Other comprehensive income (loss), net of tax	4,397	6,747
Balance at end of period	(6,048)	(10,445)	(17,192)
Accumulated Foreign Currency Adjustment Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(7,537)	(8,758)
Other comprehensive income (loss), before reclassifications, net of tax	(2,853)	1,221
Amounts reclassified from accumulated other comprehensive loss	0	0
Other comprehensive income (loss), net of tax	(2,853)	1,221
Balance at end of period	(10,390)	(7,537)	(8,758)
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(17,982)	(25,950)
Other comprehensive income (loss), before reclassifications, net of tax	1,215	7,162
Amounts reclassified from accumulated other comprehensive loss	329	806
Other comprehensive income (loss), net of tax	1,544	7,968
Balance at end of period	$ (16,438)	$ (17,982)	$ (25,950)

Accumulated Other Comprehensi_4

Accumulated Other Comprehensive Loss - Summary of Amounts Reclassified (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Actuarial (gain)	$ (101)
Tax expense
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of net loss (included within other income, net)	343	$ 1,303	$ 722
Tax expense	(14)	(497)	(276)
Amounts reclassified from accumulated other comprehensive loss	$ 329	$ 806	$ 446

Inventories, Net - Schedule of

Inventories, Net - Schedule of Company Inventories (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Inventory Disclosure [Abstract]
Purchased materials and manufactured parts, net	$ 58,572	$ 49,168
Work in process, net	21,769	17,598
Finished goods, net	141,412	133,237
Inventories, net	$ 221,753	$ 200,003

Inventories, Net - Narrative (D

Inventories, Net - Narrative (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Inventory Disclosure [Abstract]
Inventories at lower of LIFO cost or market (%)	80.00%	75.00%
Inventory, LIFO reserve	$ (26,340)	$ (4,915)
Excess and obsolete inventory reserve	$ (12,909)	$ (8,432)

Property, Plant and Equipment -

Property, Plant and Equipment - Schedule of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Property, Plant and Equipment [Line Items]
Property, plant and equipment	$ 438,792	$ 404,929
Accumulated depreciation	(225,684)	(196,310)
Property, plant and equipment, net	213,108	208,619
Land
Property, Plant and Equipment [Line Items]
Property, plant and equipment	13,295	13,296
Buildings and related improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment	108,758	105,154
Machinery and equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment	262,078	248,440
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment	7,382	6,744
Software
Property, Plant and Equipment [Line Items]
Property, plant and equipment	30,502	15,135
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment	$ 16,777	$ 16,160

Property, Plant and Equipment_2

Property, Plant and Equipment - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 34,786	$ 32,320	$ 32,779

Goodwill and Intangible Asset_2

Goodwill and Intangible Assets - Goodwill (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Goodwill [Roll Forward]
Beginning balance	$ 147,716	$ 115,829
Goodwill acquired during year	813	31,477
Exchange rate effects	301	(410)
Goodwill divested during year	(2,626)
Purchase price adjustments	24,527
Ending balance	170,129	147,716
Electrical Raceway
Goodwill [Roll Forward]
Beginning balance	108,527	76,640
Goodwill acquired during year	813	31,477
Exchange rate effects	301	(410)
Goodwill divested during year	0
Purchase price adjustments	24,527
Ending balance	133,566	108,527
Mechanical Products & Solutions
Goodwill [Roll Forward]
Beginning balance	39,189	39,189
Goodwill acquired during year	0	0
Exchange rate effects	0	0
Goodwill divested during year	(2,626)
Purchase price adjustments	0
Ending balance	$ 36,563	$ 39,189

Goodwill and Intangible Asset_3

Goodwill and Intangible Assets - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Indefinite-lived Intangible Assets [Line Items]
Useful life (years)
Amortization expense	$ 32,104	$ 22,407	$ 22,238
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, 2018	Sep. 30, 2017
Amortizable intangible assets:
Weighted Average Useful Life (Years)
Gross Carrying Value	$ 346,298	$ 377,948
Accumulated Amortization	(147,262)	(127,539)
Net Carrying Value	199,036	250,409
Intangible Assets, Net (Excluding Goodwill) [Abstract]
Gross Carrying Value	439,178	471,828
Net Carrying Value	291,916	344,289
Trade names
Indefinite-lived Intangible Assets [Line Items]
Gross carrying value/net carrying value	92,880	93,880
Customer relationships
Amortizable intangible assets:
Gross Carrying Value	330,295	350,129
Accumulated Amortization	(141,401)	(118,273)
Net Carrying Value	$ 188,894	231,856
Customer relationships | Weighted Average
Amortizable intangible assets:
Weighted Average Useful Life (Years)	11 years
Other
Amortizable intangible assets:
Gross Carrying Value	$ 16,003	27,819
Accumulated Amortization	(5,861)	(9,266)
Net Carrying Value	$ 10,142	$ 18,553
Other | Weighted Average
Amortizable intangible assets:
Weighted Average Useful Life (Years)	8 years

Goodwill and Intangible Asset_5

Goodwill and Intangible Assets - Expected Amortization Expense (Details) $ in Thousands	Sep. 30, 2018USD ($)
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
2,019	$ 31,122
2,020	29,401
2,021	29,267
2,022	27,981
2,023	27,860
2024 and thereafter	$ 53,405

Debt - Schedule of Debt (Detail

Debt - Schedule of Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Debt Instrument [Line Items]
ABL Credit Facility	$ 0	$ 85,000
Deferred financing costs	(8,194)	(4,496)
Total debt	904,247	576,078
Less: Current portion	26,561	4,215
Long-term debt	877,686	571,863
Accelerated debt repayment	18,936
Secured Debt | First Lien Term Loan Facility due December 22, 2023
Debt Instrument [Line Items]
Carrying value	912,162	495,134
Other	$ 279	$ 440

Debt - Contractual Obligation (

Debt - Contractual Obligation (Details) $ in Thousands	Sep. 30, 2018USD ($)
Debt Disclosure [Abstract]
2,019	$ 28,136
2,020	9,200
2,021	9,200
2,022	9,200
2,023	9,200
2024 and thereafter	$ 848,164

Debt - Term Loan Facilities - N

Debt - Term Loan Facilities - Narrative (Details) - USD ($) shares in Millions	Feb. 02, 2018	Dec. 22, 2016	Jan. 22, 2016	Apr. 09, 2014	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Debt Instrument [Line Items]
Loss (gain) on extinguishment of debt					$ 0	$ (9,805,000)	$ 1,661,000
Repayments of secured debt					1,217,000	641,100,000	22,175,000
Payments for repurchase of common stock					411,775,000	13,938,000	0
Debt modification costs					892,000	0	$ 0
Secured Debt | First Lien Term Loan Facility due December 22, 2023
Debt Instrument [Line Items]
Carrying value					$ 912,162,000	$ 495,134,000
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 | Redemption at 89.00%
Debt Instrument [Line Items]
Debt redeemed			$ 17,000,000
Redemption percentage of par value (%)			89.00%
Atkore International | Secured Debt | Redemption at 89.75%
Debt Instrument [Line Items]
Debt redeemed			$ 2,000,000
Redemption percentage of par value (%)			89.75%
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) (%)		$ 0.01
Atkore International | Secured Debt | New First Lien Term Loan Facility | Alternate Base Rate
Debt Instrument [Line Items]
Margin (%)		2.00%

Debt - ABL Credit Facility - Na

Debt - ABL Credit Facility - Narrative (Details) - USD ($)	Dec. 22, 2016	Dec. 22, 2016	Sep. 30, 2018	Sep. 30, 2017
Debt Instrument [Line Items]
ABL Credit Facility			$ 0	$ 85,000,000
Atkore International | Domestic Line of Credit | ABL Credit Facility | LIBOR | Minimum
Debt Instrument [Line Items]
Applicable margin (%)	1.25%
Atkore International | Domestic Line of Credit | ABL Credit Facility | LIBOR | Maximum
Debt Instrument [Line Items]
Applicable margin (%)	1.75%
Atkore International | Domestic Line of Credit | ABL Credit Facility | Alternate Base Rate | Minimum
Debt Instrument [Line Items]
Applicable margin (%)	0.25%
Atkore International | Domestic Line of Credit | ABL Credit Facility | Alternate Base Rate | Maximum
Debt Instrument [Line Items]
Applicable margin (%)	0.75%
Atkore International | Foreign Line of Credit | ABL Credit Facility | Minimum
Debt Instrument [Line Items]
Line of credit facility, unused capacity, commitment fee percentage (%)	0.25%
Atkore International | Foreign Line of Credit | ABL Credit Facility | Maximum
Debt Instrument [Line Items]
Line of credit facility, unused capacity, commitment fee percentage (%)	0.375%
Atkore International | Foreign Line of Credit | ABL Credit Facility | Bankers Acceptance Rate | Minimum
Debt Instrument [Line Items]
Applicable margin (%)	1.25%
Atkore International | Foreign Line of Credit | ABL Credit Facility | Bankers Acceptance Rate | Maximum
Debt Instrument [Line Items]
Applicable margin (%)	1.75%
Atkore International | Foreign Line of Credit | ABL Credit Facility | Prime Rate | Minimum
Debt Instrument [Line Items]
Applicable margin (%)	0.25%
Atkore International | Foreign Line of Credit | ABL Credit Facility | Prime Rate | Maximum
Debt Instrument [Line Items]
Applicable margin (%)	0.75%
Atkore International | Line of Credit | ABL Credit Facility
Debt Instrument [Line Items]
Line of credit facility, unused capacity, commitment fee percentage (%)		0.50%	0.375%
Aggregate commitments			$ 325,000,000
Credit availability			$ 315,119,000	172,994,000
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%
Fair value of amount outstanding				$ 85,000,000
Secured Debt | Atkore International | New First Lien Term Loan Facility | LIBOR
Debt Instrument [Line Items]
Applicable margin (%)	3.00%
Secured Debt | Atkore International | 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, 2018USD ($)	Sep. 30, 2018GBP (£)	Sep. 30, 2017GBP (£)	Sep. 30, 2016GBP (£)
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Forward currency contracts | £		£ 49,100,000	£ 52,600,000	£ 0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets held for sale | $	$ 650
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	6 years

Fair Value Measurements - Asset

Fair Value Measurements - Assets and Liabilities Measured at Fair Value (Details) - Recurring - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 28,175	$ 571
Forward currency contracts	0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Forward currency contracts	1,857	2,936
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
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, 2018	Sep. 30, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value	$ 913,100	$ 496,250
Fair value	916,113	498,979
First Lien Term Loan Facility due December 22, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value	913,100	496,250
Fair value	$ 916,113	$ 498,979

Commitments and Contingencies -

Commitments and Contingencies - Narrative (Details) - USD ($)	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Aug. 03, 2018	Mar. 31, 2017
Loss Contingencies [Line Items]
Purchase obligation for fiscal 2019	$ 100,649,000
Purchase obligation for fiscal 2020	330,000
Purchase obligation thereafter	46,000
Total rental expense on all operating leases	17,403,000	$ 12,941,000	$ 11,934,000
Total settlement for proir cap agreement deductible	13,000,000
Special Products Claims and Other Product Liabilities
Loss Contingencies [Line Items]
Product liability	6,755,000	$ 5,872,000
Special Products Claims and Other Product Liabilities | Minimum
Loss Contingencies [Line Items]
Probable losses	3,000,000
Special Products Claims and Other Product Liabilities | Maximum
Loss Contingencies [Line Items]
Probable losses	10,000,000
Special Products Claims
Loss Contingencies [Line Items]
Total settlement for cap agreement deductible	12,000,000
Estimate of product liability, cumulative	$ 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)

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Minimum Future Operating Lease Payments (Details) $ in Thousands	Sep. 30, 2018USD ($)
Commitments and Contingencies Disclosure [Abstract]
2,019	$ 11,069
2,020	8,226
2,021	6,580
2,022	4,886
2,023	4,254
2024 and thereafter	9,988
Total	$ 45,003

Guarantees - Narrative (Details

Guarantees - Narrative (Details) $ in Millions	Sep. 30, 2018USD ($)
Workers' compensation and general liability insurance policies
Guarantor Obligations [Line Items]
Guarantees	$ 9.9
Surety bond
Guarantor Obligations [Line Items]
Guarantees	$ 25.3

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, 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			$ 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, 2018employeesegment
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) (%)	26.00%
Number of employees affected by expiring of contracts | employee	12,000
Bargaining unit of employees | employee	400,000
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, 2018	Jun. 29, 2018	Mar. 30, 2018	Dec. 29, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 30, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Net sales	$ 477,567	$ 498,014	$ 445,000	$ 414,558	$ 395,807	$ 397,745	$ 372,791	$ 337,591	$ 1,835,139	$ 1,503,934	$ 1,523,384
Capital Expenditures									38,501	25,122	16,830
Total Assets	1,324,060				1,215,092				1,324,060	1,215,092	1,164,568
Electrical Raceway
Segment Reporting Information [Line Items]
Net sales									1,365,067	1,093,500	1,066,711
MP&S
Segment Reporting Information [Line Items]
Net sales									470,072	410,434	456,673
Intersegment Sales
Segment Reporting Information [Line Items]
Net sales									(1,625)	(1,381)	(2,067)
Intersegment Sales | Electrical Raceway
Segment Reporting Information [Line Items]
Net sales									1,544	1,283	1,919
Intersegment Sales | MP&S
Segment Reporting Information [Line Items]
Net sales									81	98	148
Operating Segments
Segment Reporting Information [Line Items]
Adjusted EBITDA									306,599	253,038	263,138
Operating Segments | Electrical Raceway
Segment Reporting Information [Line Items]
Adjusted EBITDA									255,260	189,351	181,939
Capital Expenditures									16,389	13,037	9,161
Total Assets	751,024				757,775				751,024	757,775	566,250
Operating Segments | MP&S
Segment Reporting Information [Line Items]
Adjusted EBITDA									51,339	63,687	81,199
Capital Expenditures									14,267	8,212	6,130
Total Assets	291,164				306,229				291,164	306,229	343,002
Unallocated
Segment Reporting Information [Line Items]
Capital Expenditures									7,845	3,873	1,539
Total Assets	$ 281,872				$ 151,088				$ 281,872	$ 151,088	$ 255,316

Segment Information - Reconcili

Segment Information - Reconciliation of Operating Segment Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Unallocated expenses	$ (35,050)	$ (25,430)	$ (28,136)
Depreciation and amortization	(66,890)	(54,727)	(55,017)
Interest expense, net	(40,694)	(26,598)	(41,798)
Gain (loss) on extinguishment of debt	0	(9,805)	1,661
Restructuring & impairments	(1,849)	(1,256)	(4,096)
Net periodic pension benefit cost	0	0	(441)
Stock-based compensation	(14,664)	(12,788)	(21,127)
Certain legal matters	4,833	(7,551)	(1,382)
Consulting fees	0	0	(15,425)
Transaction costs	(9,314)	(4,779)	(7,832)
Gain on sale of a business	27,575	0	0
Gain on sale of joint venture	0	(5,774)	0
Other	(4,194)	10,247	(1,103)
Impact of Fence and Sprinkler exit	0	0	811
Income before income taxes	166,352	126,125	86,781
Special Products Claims
Segment Reporting Information [Line Items]
Special products claims impact	0	0	(850)
Operating Segments
Segment Reporting Information [Line Items]
Operating segment Adjusted EBITDA	306,599	253,038	263,138
Operating Segments | Electrical Raceway
Segment Reporting Information [Line Items]
Operating segment Adjusted EBITDA	255,260	189,351	181,939
Operating Segments | MP&S
Segment Reporting Information [Line Items]
Operating segment Adjusted EBITDA	$ 51,339	$ 63,687	$ 81,199

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, 2018	Jun. 29, 2018	Mar. 30, 2018	Dec. 29, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 30, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	$ 214,715				$ 215,671				$ 214,715	$ 215,671	$ 209,936
Net sales	477,567	$ 498,014	$ 445,000	$ 414,558	395,807	$ 397,745	$ 372,791	$ 337,591	1,835,139	1,503,934	1,523,384
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	201,101				202,823				201,101	202,823	204,640
Net sales									1,651,636	1,367,907	1,395,750
Other Americas
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	138				164				138	164	175
Net sales									43,013	37,908	40,573
Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	11,090				9,306				11,090	9,306	1,295
Net sales									90,915	55,181	40,246
Asia-Pacific
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	$ 2,386				$ 3,378				2,386	3,378	3,826
Net sales									$ 49,575	$ 42,938	$ 46,815

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, 2018	Jun. 29, 2018	Mar. 30, 2018	Dec. 29, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 30, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Revenue from External Customer [Line Items]
Net sales	$ 477,567	$ 498,014	$ 445,000	$ 414,558	$ 395,807	$ 397,745	$ 372,791	$ 337,591	$ 1,835,139	$ 1,503,934	$ 1,523,384
Electrical Raceway
Revenue from External Customer [Line Items]
Net sales									1,365,067	1,093,500	1,066,711
Electrical Raceway | Metal Electrical Conduit and Fittings
Revenue from External Customer [Line Items]
Net sales									517,935	349,239	331,526
Electrical Raceway | Armored Cable and Fittings
Revenue from External Customer [Line Items]
Net sales									336,388	323,070	318,279
Electrical Raceway | PVC Electrical Conduit & Fittings
Revenue from External Customer [Line Items]
Net sales									311,811	265,389	258,954
Electrical Raceway | Other raceway products
Revenue from External Customer [Line Items]
Net sales									198,933	155,802	157,952
Mechanical Products & Solutions
Revenue from External Customer [Line Items]
Net sales									470,072	410,434	456,673
Mechanical Products & Solutions | Mechanical Pipe
Revenue from External Customer [Line Items]
Net sales									253,381	211,245	249,473
Mechanical Products & Solutions | Other MP&S products
Revenue from External Customer [Line Items]
Net sales									216,691	199,189	199,384
Mechanical Products & Solutions | Impact of Fence and Sprinkler
Revenue from External Customer [Line Items]
Net sales									$ 0	$ 0	$ 7,816

Quarterly Operating Results (_2

Quarterly Operating Results (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 22, 2017	Sep. 30, 2018	Jun. 29, 2018	Mar. 30, 2018	Dec. 29, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 30, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Aug. 03, 2018
Debt Instrument [Line Items]
Net sales		$ 477,567	$ 498,014	$ 445,000	$ 414,558	$ 395,807	$ 397,745	$ 372,791	$ 337,591	$ 1,835,139	$ 1,503,934	$ 1,523,384
Gross profit		111,731	120,329	109,157	96,867	89,513	92,485	87,609	91,665	438,084	361,270	367,576
Net income		$ 32,699	$ 34,199	$ 42,558	$ 27,189	$ 20,857	$ 27,465	$ 18,935	$ 17,382	$ 136,645	$ 84,639	$ 58,796
Net income per share
Basic (in dollars per share)		$ 0.69	$ 0.73	$ 0.83	$ 0.43	$ 0.33	$ 0.43	$ 0.30	$ 0.28	$ 2.59	$ 1.33	$ 0.94
Diluted (in dollars per share)		$ 0.66	$ 0.70	$ 0.79	$ 0.41	$ 0.31	$ 0.41	$ 0.28	$ 0.26	$ 2.48	$ 1.27	$ 0.94
Gain (loss) on extinguishment of debt										$ 0	$ (9,805)	$ 1,661
Proceeds from divestiture of interest in joint venture								$ 5,774
Effective income tax rate reconciliation, effect of Tax Cuts and Jobs Act of 2017, amount	$ 4,758									708
Gain on sale of business, pre-tax										$ (42,631)	$ 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, pre-tax			$ 26,737
Gain on sale of business, working capital adjustment			$ 838
Restatement Adjustment | Antidumping Duty, Malleable Iron Pipe Fittings from China
Net income per share
Product liability													$ (7,171)

Schedule I - Condensed Financ_2

Schedule I - Condensed Financial Information - Balance Sheets (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Sep. 25, 2015
Assets
Total Assets	$ 1,324,060	$ 1,215,092	$ 1,164,568
Liabilities and Equity
Total Liabilities	1,202,001	854,221
Equity:
Common stock, $0.01 par value, 1,000,000,000 shares authorized, 47,079,645 and 63,305,434 shares issued and outstanding, respectively	472	634
Treasury stock, held at cost, 260,900 and 260,900 shares, respectively	(2,580)	(2,580)
Additional paid-in capital	457,978	423,232
Accumulated deficit	(317,373)	(42,433)
Accumulated other comprehensive loss	(16,438)	(17,982)
Total Equity	122,059	360,871	$ 257,246	$ 156,277
Total Liabilities and Equity	1,324,060	1,215,092
Atkore International Group Inc.
Assets
Investment in subsidiary	122,059	360,871
Total Assets	122,059	360,871
Liabilities and Equity
Total Liabilities	0	0
Equity:
Common stock, $0.01 par value, 1,000,000,000 shares authorized, 47,079,645 and 63,305,434 shares issued and outstanding, respectively	472	634
Treasury stock, held at cost, 260,900 and 260,900 shares, respectively	(2,580)	(2,580)
Additional paid-in capital	457,978	423,232
Accumulated deficit	(317,373)	(42,433)
Accumulated other comprehensive loss	(16,438)	(17,982)
Total Equity	122,059	360,871
Total Liabilities and Equity	$ 122,059	$ 360,871

Schedule I - Condensed Financ_3

Schedule I - Condensed Financial Information - Balance Sheets (Parenthetical) (Details) - $ / shares	Sep. 30, 2018	Sep. 30, 2017
Condensed Balance Sheet Statements, Captions [Line Items]
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (shares)	1,000,000,000	1,000,000,000
Common stock, shares issued (shares)	47,079,645	63,305,434
Common stock, shares outstanding (shares)	47,079,645	63,305,434
Treasury stock (shares)	260,900	260,900
Atkore International Group Inc.
Condensed Balance Sheet Statements, Captions [Line Items]
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (shares)	1,000,000,000	1,000,000,000
Common stock, shares issued (shares)	47,079,645	63,305,434
Common stock, shares outstanding (shares)	47,079,645	63,305,434
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, 2018	Jun. 29, 2018	Mar. 30, 2018	Dec. 29, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 30, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Condensed Income Statements, Captions [Line Items]
Net income	$ 32,699	$ 34,199	$ 42,558	$ 27,189	$ 20,857	$ 27,465	$ 18,935	$ 17,382	$ 136,645	$ 84,639	$ 58,796
Other comprehensive income (loss) of subsidiary, net of tax									1,544	7,968	(4,917)
Comprehensive income									138,189	92,607	53,879
Atkore International Group Inc.
Condensed Income Statements, Captions [Line Items]
Net income									136,645	84,639	58,796
Other comprehensive income (loss) of subsidiary, net of tax									1,544
Comprehensive income									$ 138,189	$ 92,607	$ 53,879

Schedule I - Condensed Financ_5

Schedule I - Condensed Financial Information - Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Cash Flows from Operating Activities:
Net cash provided by operating activities	$ 145,703	$ 121,654	$ 156,646
Cash Flows from Investing Activities:
Net cash provided by (used for) investing activities	2,514	(205,833)	(12,895)
Cash Flows from Financing Activities:
Issuance of common stock	20,110	12,168	52
Repurchase of common stock	(411,775)	(13,938)	0
Net cash (used for) financing activities	(65,931)	(67,760)	(23,908)
Increase (decrease) in cash and cash equivalents	80,944	(154,561)	119,681
Cash and cash equivalents:
Cash and cash equivalents at beginning of period	45,718	200,279	80,598
Cash and cash equivalents at end of period	126,662	45,718	200,279
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	411,775	13,938	0
Distribution paid to subsidiary	(20,110)	(12,168)	(52)
Net cash provided by (used for) investing activities	391,665	1,770	(52)
Cash Flows from Financing Activities:
Issuance of common stock		12,168	52
Repurchase of common stock	(411,775)	(13,938)	0
Net cash (used for) financing activities	(391,665)	(1,770)	52
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) $ in Thousands, shares in Millions	May 27, 2016	Mar. 06, 2014USD ($)shares	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)	Dec. 22, 2010
Condensed Financial Statements, Captions [Line Items]
Payment to redeem shares		$ 250,000
Stock split of common stock	1.37
Atkore International Group Inc.
Condensed Financial Statements, Captions [Line Items]
Distribution received from subsidiary			$ 411,775	$ 13,938	$ 0
Common Stock
Condensed Financial Statements, Captions [Line Items]
Non-binding letter of intent to acquire common stock (shares) | shares		40.3
Expense related to share redemption		$ 2,000
Atkore International | CD&R
Condensed Financial Statements, Captions [Line Items]
Preferred stock, percentage of outstanding capital stock (on an as-converted basis) (%)						51.00%
Common stock, percentage of outstanding capital stock (%)						49.00%

Schedule II - Valuation and Q_2

Schedule II - Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Movement in Valuation Allowances and Reserves [Roll Forward]
Deferred Tax Assets, Valuation Allowance	$ 7,427	$ 9,512
Accounts Receivable Allowance for Doubtful Accounts
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	(1,239)	(1,006)	$ (1,173)
Additional (Charges)/Benefit to Income	(389)	(243)	(426)
Write offs and Other	(134)	10	593
Balance at End of Year	(1,762)	(1,239)	(1,006)
Deferred Tax Valuation Allowance
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	(9,512)	(8,658)	(7,532)
Additional (Charges)/Benefit to Income	428	(1,164)	(2,604)
Write offs and Other	$ 1,657	310	1,478
Balance at End of Year		$ (9,512)	$ (8,658)