Summary of Significant Accounting Policies (Policies)	9 Months Ended
Summary of Significant Accounting Policies (Policies)	Jun. 30, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The Consolidated Financial Statements include the accounts of Star Group, L.P. and its subsidiaries. All material intercompany items and transactions have been eliminated in consolidation. The financial information included herein is unaudited; however, such information reflects all adjustments (consisting solely of normal recurring adjustments), which are, in the opinion of management, necessary for the fair statement of financial condition and results for the interim periods. Due to the seasonal nature of the Company’s business, the results of operations and cash flows for the nine month period ended June 30, 2018 are not necessarily indicative of the results to be expected for the full year. These interim financial statements of the Company have been prepared in accordance with U.S. Generally Accepted Accounting Principles (“GAAP”) for interim financial information and Rule 10-01 of Regulation S-X of the U.S. Securities and Exchange Commission and should be read in conjunction with the financial statements included in the Company’s Annual Report on Form 10-K for the year ended September 30, 2017.
Comprehensive Income (Loss)	Comprehensive Income (Loss) Comprehensive income (loss) is comprised of Net income (loss) and Other comprehensive income (loss). Other comprehensive income (loss) consists of the unrealized gain amortization on the Company’s pension plan obligation for its two frozen defined benefit pension plans, unrealized loss on captive insurance collateral, and the corresponding tax effect.
Cash, Cash Equivalents, and Restricted Cash	Cash, Cash Equivalents, and Restricted Cash The Company considers all highly liquid investments with an original maturity of three months or less, when purchased, to be cash equivalents. At June 30, 2018, the $9.7 million of cash, cash equivalents, and restricted cash on the condensed consolidated statement of cash flows is composed of $9.4 million of cash and cash equivalents and $0.3 million of restricted cash. At September 30, 2017, the $52.7 million of cash, cash equivalents, and restricted cash on the condensed consolidated statements of cash flow is composed of $52.5 million of cash and cash equivalents and $0.3 million of restricted cash. Restricted cash represents deposits held by our captive insurance company that are required by state insurance regulations to remain in the captive insurance company as cash.
Captive Insurance Collateral	Captive Insurance Collateral At June 30, 2018, captive insurance collateral is comprised of $44.5 million of Level 1 debt securities measured at fair value and $0.7 million of mutual funds measured at net asset value. At September 30, 2017, the balance was comprised of $11.3 million of Level 1 debt securities measured at fair value and $0.5 million of mutual funds measured at net asset value. Unrealized gains and losses, net of related income taxes, are reported as accumulated other comprehensive loss, except for losses from impairments which are determined to be other-than-temporary. Realized gains and losses, and declines in value judged to be other-than-temporary on available-for-sale securities are included in the determination of net income and are included in Interest expense, net, at which time the average cost basis of these securities are adjusted to fair value. The investments are held by our captive insurance company in an irrevocable trust as collateral for certain workers’ compensation, general and automobile liability claims. The collateral is required by a third party insurance carrier that insures per claim amounts above a set deductible. Due to the expected timing of claim payments, the nature of the collateral agreement with the carrier, and our captive insurance company’s source of other operating cash, the collateral is not expected to be used to pay obligations within the next twelve months. At September 30, 2017, the investments were held for workers’ compensation, general and automobile liability claims incurred and expected to be incurred in fiscal 2017. In the first quarter of fiscal 2018 we deposited $34.2 million of cash into the irrevocable trust to secure certain workers’ compensation, general and automobile liability claims incurred and expected to be incurred from fiscal 2004 to fiscal 2016 and fiscal 2018.
Weather Hedge Contract	Weather Hedge Contract To partially mitigate the adverse effect of warm weather on cash flows, the Company has used weather hedge contracts for a number of years. Weather hedge contracts are recorded in accordance with the intrinsic value method defined by the Financial Accounting Standards Board (“FASB”) Accounting Standards Codification (“ASC”) 815-45-15 Derivatives and Hedging, Weather Derivatives (EITF 99-2). The premium paid is included in the caption Prepaid expenses and other current assets in the accompanying balance sheets and amortized over the life of the contract, with the intrinsic value method applied at each interim period. The Company has weather hedge contracts for fiscal years 2018, 2019, 2020 and 2021. Under these contracts, we are entitled to receive a payment if the total number of degree days within the hedge period is less than the prior ten year average. The “Payment Thresholds,” or strikes, are set at various levels. In addition, we will be obligated to make a payment capped at $5.0 million if degree days exceed the prior ten year average. The hedge period runs from November 1 through March 31, taken as a whole, for each respective fiscal year. For fiscal 2019, 2020 and 2021 the maximum that the Company can receive is $12.5 million and the maximum that the Company would be obligated to pay is $5.0 million. As of June 30, 2018, the Company recorded a charge of $1.9 million under this contract that increased delivery and branch expenses. The amount was paid in April 2018. No charge or benefit was recorded as of June 30, 2017.
New England Teamsters and Trucking Industry Pension Fund ("the NETTI Fund") Liability	New England Teamsters and Trucking Industry Pension Fund (“the NETTI Fund”) Liability As of June 30, 2018, we had $0.2 million and $17.2 million balances included in the captions Accrued expenses and other current liabilities and Other long-term liabilities, respectively, on our condensed consolidated balance sheet representing the remaining balance of the NETTI Fund withdrawal liability. Based on the borrowing rates currently available to the Company for long-term financing of a similar maturity, the fair value of the NETTI Fund withdrawal liability as of June 30, 2018 was $21.1 million. We utilized Level 2 inputs in the fair value hierarchy of valuation techniques to determine the fair value of this liability.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements In July 2015, the FASB issued ASU No. 2015-11, Simplifying the Measurement of Inventory. The update changes the measurement principle for inventory from the lower of cost or market to the lower of cost and net realizable value. The Company adopted the ASU effective December 31, 2017. The adoption of ASU No. 2015-11 did not have an impact on the Company’s consolidated financial statements and related disclosures.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers, which requires an entity to recognize the amount of revenue to which it expects to be entitled for the transfer of promised goods or services to customers. The FASB has also issued several updates to ASU 2014-09. This ASU will replace most existing revenue recognition guidance in GAAP when it becomes effective. This new guidance is effective for our annual reporting period beginning in the first quarter of fiscal 2019, with early adoption permitted beginning in the first quarter of fiscal 2018. The standard permits the use of either the retrospective or cumulative effect transition method. The Company is in the process of evaluating the effect that ASU 2014-09 will have on its revenue streams, consolidated financial statements and related disclosures. The Company has not yet selected a transition method, nor does it intend to early adopt. In February 2016, the FASB issued ASU No. 2016-02, Leases. The update requires all leases with a term greater than twelve months to be recognized on the balance sheet by calculating the discounted present value of such leases and accounting for them through a right-of-use asset and an offsetting lease liability, and the disclosure of key information pertaining to leasing arrangements. This new guidance is effective for our annual reporting period beginning in the first quarter of fiscal 2020, with early adoption permitted. The Company does not intend to early adopt. The Company is continuing to evaluate the effect that ASU No. 2016-02 could have on its consolidated financial statements and related disclosures, but has not yet selected a transition method. The new guidance will materially change how we account for operating leases for office space, trucks and other equipment. Upon adoption, we expect to recognize discounted right-of-use assets and offsetting lease liabilities related to our operating leases of office space, trucks and other equipment. As of June 30, 2018, the undiscounted future minimum lease payments through 2032 for such operating leases are approximately $130.3 million, but the amount of leasing activity expected between June 30, 2018, and the date of adoption, is currently unknown. For this reason we are unable to estimate the discounted right-of-use assets and lease liabilities as of the date of adoption. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses. The update broadens the information that an entity should consider in developing expected credit loss estimates, eliminates the probable initial recognition threshold, and allows for the immediate recognition of the full amount of expected credit losses. This new guidance is effective for our annual reporting period beginning in the first quarter of fiscal 2021, with early adoption permitted in the first quarter of fiscal 2020. The Company is evaluating the effect that ASU No. 2016-13 will have on its consolidated financial statements and related disclosures, but has not yet determined the timing of adoption. In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flow (Topic 230): Classification of Certain Cash Receipts and Cash Payments. The update addresses the issues of debt prepayment or debt extinguishment costs, settlement of zero-coupon debt instruments or other debt instruments with coupon interest rates that are insignificant in relation to the effective interest rate of the borrowing, contingent consideration payments made after a business combination, proceeds from the settlement of insurance claims, proceeds from the settlement of corporate owned life insurance policies, distributions received from equity method investees, beneficial interests in securitization transactions, and separately identifiable cash flows and application of the predominance principle. This new guidance is effective for our annual reporting period beginning in the first quarter of fiscal 2019, with early adoption permitted. The Company has not determined the timing of adoption, but does not expect ASU 2016-15 to have a material impact on its consolidated financial statements and related disclosures. In January 2017, the FASB issued ASU No. 2017-01, Business Combinations (Topic 805): Clarifying the definition of a business. The update clarifies the definition of a business with the objective of adding guidance to assist entities with evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. This new guidance is effective for our annual reporting period beginning in the first quarter of fiscal 2019, with early adoption permitted. The Company has not determined the timing of adoption, but does not expect ASU 2017-01 to have a material impact on its consolidated financial statements and related disclosures. In January 2017, the FASB issued ASU No. 2017-04, Intangibles – Goodwill and Other (Topic 230): Simplifying the test for goodwill impairment. The update simplifies how an entity is required to test goodwill for impairment. An entity should recognize an impairment charge for the amount by which the carrying amount exceeds the reporting unit’s fair value, but not exceed the total amount of goodwill allocated to the reporting unit. This new guidance is effective for our annual reporting period beginning in the first quarter of fiscal 2021, with early adoption permitted. The Company has not determined the timing of adoption, but does not expect ASU 2017-04 to have a material impact on its consolidated financial statements and related disclosures. In February 2018, the FASB issued ASU No. 2018-02, Income Statement – Reporting Comprehensive Income, which allow a reclassification from accumulated other comprehensive income to retained earnings for stranded tax effects resulting from the Tax Cuts and Jobs Act. This new guidance is effective for our annual reporting period beginning in the first quarter of fiscal 2020, with early adoption permitted. The Company is evaluating the effect that ASU No. 2018-02 will have on its consolidated financial statements and related disclosures, but has not determined the timing of adoption.

Disclosure of accounting policy for investments in financial assets, including marketable securities (debt and equity securities with readily determinable fair values), investments accounted for under the equity method and cost method, securities borrowed and loaned, and repurchase and resale agreements. For marketable securities, the disclosure may include the entity's accounting treatment for transfers between investment categories and how the fair values for such securities are determined. Also, for all investments, an entity may describe its policy for assessing, recognizing and measuring impairment of the investment.

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Disclosure of accounting policy for pension and other postretirement benefit plans. This accounting policy may address (1) the types of plans sponsored by the entity, and the benefits provided by each plan (2) groups that participate in (or are covered by) each plan (3) how plan assets, liabilities and expenses are measured, including the use of any actuaries and (4) significant assumptions used by the entity to value plan assets and liabilities and how such assumptions are derived.

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