Summary of Significant Accounting Policies (Policies)	6 Months Ended
Mar. 31, 2020
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The Company prepared the condensed consolidated financial statements in accordance with U.S. generally accepted accounting principles (“GAAP”) for interim financial information and the requirements of the Securities and Exchange Commission (“SEC”). As permitted under those rules, certain footnotes or other financial information have been condensed or omitted. Certain prior period amounts have been reclassified to conform to current period presentation. The balance sheet as of March 31, 2019 has been presented for a better understanding of the impact of seasonal fluctuations on the Company’s financial condition. In management’s opinion, the financial statements include all normal and recurring adjustments that are considered necessary for the fair presentation of the Company’s financial position and operating results. The results for the three and six months ended March 31, 2020 are not necessarily indicative of the results to be expected for the twelve months ending September 30, 2020 (“fiscal year 2020” or “2020”). The three-month periods ended March 31, 2020 and 2019 had 64 and 63 business days, respectively. The six-month periods ended March 31, 2020 and 2019 had 126 and 125 business days, respectively. These interim Condensed Consolidated Financial Statements should be read in conjunction with the audited Consolidated Financial Statements and Notes thereto contained in the Company’s fiscal year 2019 (“2019”) Annual Report on Form 10-K for the year ended September 30, 2019.
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in these consolidated financial statements and accompanying notes. Significant items subject to such estimates include accounts receivable, inventories, purchase price allocations, goodwill and intangibles, and income taxes. Assumptions made in the development of these estimates contemplate the impact of the novel coronavirus (“COVID‑19”) on the economy and our anticipated results; however, actual amounts could differ from these estimates.
Recent Accounting Pronouncements - Adopted and Not Yet Adopted	Recent Accounting Pronouncements—Adopted In February 2016, the FASB issued ASU 2016-02, “Leases.” This guidance replaces most existing accounting for leases and requires enhanced disclosures. The guidance requires the Company to record a right-of-use asset and a lease liability for most of the Company’s leases, including those previously treated as operating leases. The Company adopted the standard using the modified retrospective transition method as of October 1, 2019 and will not apply the standard to comparative prior periods presented. The Company used the package of transition practical expedients outlined in the transition guidance. The most significant effects of the new standard were the recognition of $483.5 million of operating lease assets and $476.0 million of operating lease liabilities on October 1, 2019. As part of the adoption, the Company carried forward the assessment from the previous lease standard of whether Beacon’s contracts contain (or are) leases, the classification of leases, and remaining lease terms. The accounting for finance leases remains unchanged. The adoption of the new standard did not have a material impact on the Company’s consolidated results of operations or cash flows. See Note 8 for further discussion. In February 2018, the FASB issued ASU 2018-02, “Income Statement – Reporting Comprehensive Income.” This guidance is intended to address the accounting treatment for the tax effects on items within accumulated other comprehensive income as a result of the adoption of the Tax Cuts and Jobs Act of 2017. This new standard became effective for the Company on October 1, 2019. The adoption of this new guidance did not have a material impact on the Company’s financial statements and related disclosures. Recent Accounting Pronouncements—Not Yet Adopted In June 2016, the FASB issued ASU 2016-13, “Financial Instruments—Credit Losses: Measurement of Credit Losses on Financial Instruments.” This guidance is intended to introduce a revised approach to the recognition and measurement of credit losses, emphasizing an updated model based on expected losses rather than incurred losses. This new standard is effective for annual reporting periods, and interim reporting periods contained therein, beginning after December 15, 2019, and early adoption is permitted. The Company is currently evaluating the impact that this guidance may have on its financial statements and related disclosures. In January 2017, the FASB issued ASU 2017-04, “Simplifying the Accounting for Goodwill Impairment.” This guidance is intended to introduce a simplified approach to measurement of goodwill impairment, eliminating the need for a hypothetical purchase price allocation and instead measuring impairment by the amount a reporting unit’s carrying value exceeds its fair value. This new standard is effective for annual reporting periods, and interim reporting periods contained therein, beginning after December 15, 2019, and early adoption is permitted. The Company does not expect the adoption of this new guidance to have a material impact on its financial statements and related disclosures. In December 2019, the FASB issued ASU 2019-12, “Income Taxes – Simplifying the Accounting for Income Taxes.” This guidance is intended to simplify the accounting for income taxes by removing certain exceptions, clarifying existing guidance and improving consistent application of the guidance. This new standard is effective for annual reporting periods, and interim reporting periods contained therein, beginning after December 15, 2020, and early adoption is permitted. The Company is currently evaluating the impact that this guidance may have on its financial statements and related disclosures. In March 2020, the FASB issued ASU 2020-04, “Reference Rate Reform (Topic 848), Facilitation of the Effects of Reference Rate Reform on Financial Reporting.” The guidance provides optional practical expedients to ease the potential burden in accounting for contract modifications and hedge accounting related to reference rate reform. The standard is effective as of March 12, 2020 through December 31, 2022. However, the standard is not applicable to contract modifications made and hedging relationships entered into or evaluated after December 31, 2022. The Company expects to elect optional expedients and exceptions provided by the guidance, as needed, related to the 2023 ABL and 2025 Term Loan debt instruments, both of which include interest rates based on a LIBOR rate (with a floor) plus a fixed spread. The Company will evaluate and disclose the impact of this guidance in the period of election, as well as the nature and reason for doing so.

Summary of Significant Accounting Policies (Policies)

6 Months Ended

Mar. 31, 2020

The Company prepared the condensed consolidated financial statements in accordance with U.S. generally accepted accounting principles (“GAAP”) for interim financial information and the requirements of the Securities and Exchange Commission (“SEC”). As permitted under those rules, certain footnotes or other financial information have been condensed or omitted. Certain prior period amounts have been reclassified to conform to current period presentation. The balance sheet as of March 31, 2019 has been presented for a better understanding of the impact of seasonal fluctuations on the Company’s financial condition.

In management’s opinion, the financial statements include all normal and recurring adjustments that are considered necessary for the fair presentation of the Company’s financial position and operating results. The results for the three and six months ended March 31, 2020 are not necessarily indicative of the results to be expected for the twelve months ending September 30, 2020 (“fiscal year 2020” or “2020”).

The three-month periods ended March 31, 2020 and 2019 had 64 and 63 business days, respectively. The six-month periods ended March 31, 2020 and 2019 had 126 and 125 business days, respectively.

These interim Condensed Consolidated Financial Statements should be read in conjunction with the audited Consolidated Financial Statements and Notes thereto contained in the Company’s fiscal year 2019 (“2019”) Annual Report on Form 10-K for the year ended September 30, 2019.

Use of Estimates

The preparation of consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in these consolidated financial statements and accompanying notes. Significant items subject to such estimates include accounts receivable, inventories, purchase price allocations, goodwill and intangibles, and income taxes. Assumptions made in the development of these estimates contemplate the impact of the novel coronavirus (“COVID‑19”) on the economy and our anticipated results; however, actual amounts could differ from these estimates.

Recent Accounting Pronouncements - Adopted and Not Yet Adopted

Recent Accounting Pronouncements—Adopted

In February 2016, the FASB issued ASU 2016-02, “Leases.” This guidance replaces most existing accounting for leases and requires enhanced disclosures. The guidance requires the Company to record a right-of-use asset and a lease liability for most of the Company’s leases, including those previously treated as operating leases. The Company adopted the standard using the modified retrospective transition method as of October 1, 2019 and will not apply the standard to comparative prior periods presented. The Company used the package of transition practical expedients outlined in the transition guidance. The most significant effects of the new standard were the recognition of $483.5 million of operating lease assets and $476.0 million of operating lease liabilities on October 1, 2019. As part of the adoption, the Company carried forward the assessment from the previous lease standard of whether Beacon’s contracts contain (or are) leases, the classification of leases, and remaining lease terms. The accounting for finance leases remains unchanged. The adoption of the new standard did not have a material impact on the Company’s consolidated results of operations or cash flows. See Note 8 for further discussion.

In February 2018, the FASB issued ASU 2018-02, “Income Statement – Reporting Comprehensive Income.” This guidance is intended to address the accounting treatment for the tax effects on items within accumulated other comprehensive income as a result

of the adoption of the Tax Cuts and Jobs Act of 2017. This new standard became effective for the Company on October 1, 2019. The adoption of this new guidance did not have a material impact on the Company’s financial statements and related disclosures.

Recent Accounting Pronouncements—Not Yet Adopted

In June 2016, the FASB issued ASU 2016-13, “Financial Instruments—Credit Losses: Measurement of Credit Losses on Financial Instruments.” This guidance is intended to introduce a revised approach to the recognition and measurement of credit losses, emphasizing an updated model based on expected losses rather than incurred losses. This new standard is effective for annual reporting periods, and interim reporting periods contained therein, beginning after December 15, 2019, and early adoption is permitted. The Company is currently evaluating the impact that this guidance may have on its financial statements and related disclosures.

In January 2017, the FASB issued ASU 2017-04, “Simplifying the Accounting for Goodwill Impairment.” This guidance is intended to introduce a simplified approach to measurement of goodwill impairment, eliminating the need for a hypothetical purchase price allocation and instead measuring impairment by the amount a reporting unit’s carrying value exceeds its fair value. This new standard is effective for annual reporting periods, and interim reporting periods contained therein, beginning after December 15, 2019, and early adoption is permitted. The Company does not expect the adoption of this new guidance to have a material impact on its financial statements and related disclosures.

In December 2019, the FASB issued ASU 2019-12, “Income Taxes – Simplifying the Accounting for Income Taxes.” This guidance is intended to simplify the accounting for income taxes by removing certain exceptions, clarifying existing guidance and improving consistent application of the guidance. This new standard is effective for annual reporting periods, and interim reporting periods contained therein, beginning after December 15, 2020, and early adoption is permitted. The Company is currently evaluating the impact that this guidance may have on its financial statements and related disclosures.

In March 2020, the FASB issued ASU 2020-04, “Reference Rate Reform (Topic 848), Facilitation of the Effects of Reference Rate Reform on Financial Reporting.” The guidance provides optional practical expedients to ease the potential burden in accounting for contract modifications and hedge accounting related to reference rate reform. The standard is effective as of March 12, 2020 through December 31, 2022. However, the standard is not applicable to contract modifications made and hedging relationships entered into or evaluated after December 31, 2022. The Company expects to elect optional expedients and exceptions provided by the guidance, as needed, related to the 2023 ABL and 2025 Term Loan debt instruments, both of which include interest rates based on a LIBOR rate (with a floor) plus a fixed spread. The Company will evaluate and disclose the impact of this guidance in the period of election, as well as the nature and reason for doing so.