Summary of Significant Accounting Policies (Policies)	3 Months Ended
Dec. 31, 2018
Accounting Policies [Abstract]
Basis of Presentation	BASIS OF PRESENTATION – These notes are an integral part of the accompanying unaudited financial statements of Spire Inc. presented on a consolidated basis (“Spire” or the “Company”), Spire Missouri Inc. (“Spire Missouri” or the “Missouri Utilities”) and Spire Alabama Inc. (“Spire Alabama”). Spire Missouri and Spire Alabama are wholly owned subsidiaries of the Company. Spire Missouri, Spire Alabama and the subsidiaries of Spire EnergySouth Inc. (“Spire EnergySouth”) are collectively referred to as the “Utilities.” The subsidiaries of Spire EnergySouth are Spire Gulf Inc. and Spire Mississippi Inc. The accompanying unaudited financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) for interim financial information with the instructions to Form 10-Q and Rule 10-01 of Regulation S‑X. Accordingly, they do not include all the disclosures required for complete financial statements. In the opinion of management, the accompanying unaudited financial statements include all adjustments (consisting of only normal recurring adjustments) necessary for the fair presentation of the results of operations for the periods presented. This Form 10-Q should be read in conjunction with the Notes to Financial Statements contained in Spire’s, Spire Missouri’s and Spire Alabama’s combined Annual Report on Form 10-K for the fiscal year ended September 30, 2018. The consolidated financial position, results of operations, and cash flows of Spire include the accounts of the Company and all its subsidiaries. Transactions and balances between consolidated entities have been eliminated from the consolidated financial statements of Spire. In compliance with GAAP, transactions between Spire Missouri and Spire Alabama and their affiliates, as well as intercompany balances on their balance sheets, have not been eliminated from their separate financial statements.
Derivatives	DERIVATIVES – In the course of their business, certain subsidiaries of the Company enter into commitments associated with the purchase or sale of natural gas. Certain of their derivative natural gas contracts are designated as normal purchases or normal sales and, as such, are excluded from the scope of Financial Accounting Standards Board (“FASB”) Accounting Standards Codification (“ASC”) Topic 815, Derivatives and Hedging. Those contracts are accounted for as executory contracts and recorded on an accrual basis. Revenues and expenses from such contracts are recorded using a gross presentation. Contracts not designated as normal purchases or normal sales are recorded as derivatives with changes in fair value recognized in earnings in the periods prior to physical delivery. Certain of Spire Marketing’s wholesale purchase and sale transactions are classified as trading activities for financial reporting purposes. Under GAAP, revenues and expenses associated with trading activities are presented on a net basis in Gas Marketing operating revenues (or expenses, if negative) in the Condensed Consolidated Statements of Income. This net presentation has no effect on operating income or net income.
Regulated Operations	REGULATED OPERATIONS – The Utilities account for their regulated operations in accordance with FASB ASC Topic 980, Regulated Operations. This topic sets forth the application of GAAP for those companies whose rates are established by or are subject to approval by an independent third-party regulator. The provisions of this accounting guidance require, among other things, that financial statements of a regulated enterprise reflect the actions of regulators, where appropriate. These actions may result in the recognition of revenues and expenses in time periods that are different than non-regulated enterprises. When this occurs, costs are deferred as assets in the balance sheet (regulatory assets) and recorded as expenses when those amounts are reflected in rates. In addition, regulators can impose liabilities upon a regulated company for amounts previously collected from customers and for recovery of costs that are expected to be incurred in the future (regulatory liabilities). Management believes that the current regulatory environment supports the continued use of these regulatory accounting principles and that all regulatory assets and regulatory liabilities are recoverable or refundable through the regulatory process. As authorized by the Missouri Public Service Commission (“MoPSC”), the Mississippi Public Service Commission (“MSPSC”) and the Alabama Public Service Commission (“APSC”), the Purchased Gas Adjustment (“PGA”) clauses and Gas Supply Adjustment (“GSA”) riders allow the Utilities to pass through to customers the cost of purchased gas supplies. Regulatory assets and liabilities related to the PGA clauses and the GSA riders are both labeled Unamortized Purchased Gas Adjustments herein. See additional information about regulatory assets and liabilities in Note 4, Regulatory Matters.
Transactions with Affiliates	TRANSACTIONS WITH AFFILIATES – Transactions between affiliates of the Company have been eliminated from the consolidated statements of Spire. Spire Missouri and Spire Alabama borrowed funds from the Company and incurred related interest, as reflected in their separate financial statements, and they participated in normal intercompany shared services transactions
Accrued Capital Expenditures	Accrued capital expenditures, shown in the following table, are excluded from capital expenditures in the statements of cash flows until paid.
New Accounting Pronouncements	NEW ACCOUNTING PRONOUNCEMENTS – Spire, Spire Missouri and Spire Alabama adopted the guidance in Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers, and related amendments (collectively, “ASC 606”), in the first quarter of fiscal year 2019 using the modified retrospective method applied to all contracts at October 1, 2018. The core principle of ASC 606 is that revenue should be recognized to depict the transfer of control of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. Under the new standard, an entity must identify the performance obligations in a contract, determine the transaction price and allocate the price to specific performance obligations to recognize revenue when the obligation is completed. In addition, ASC 606 requires disclosure of the nature, amount, timing and uncertainty of revenue and cash flows arising from contracts with customers. The adoption of ASC 606 did not result in significant changes to how the Spire, Spire Missouri and Spire Alabama recognize revenue, and therefore, no cumulative effect adjustment to the opening balance of retained earnings was required, and there was no significant impact to financial results after adoption. The adoption did result in changes to the disclosures about revenue, which are included in Note 2, Revenue. Some revenue arrangements, such as alternative revenue programs and certain derivative contracts, are excluded from the scope of ASC 606 and, therefore, are presented separately in disclosures. Also effective October 1, 2018, Spire, Spire Missouri and Spire Alabama adopted ASU No. 2017-07, Compensation – Retirement Benefits: Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost, with no material impact on interim or annual financial statements. The amended guidance requires that the service cost component of net periodic pension and postretirement benefit costs be presented within the same line item in the income statement as other compensation costs (except for the amount being capitalized), while other components are to be presented outside the subtotal of operating income and are no longer eligible for capitalization (e.g., as part of utility plant). The amended guidance is applied retrospectively for income statement presentation and prospectively for capitalization. The Company, Spire Missouri and Spire Alabama elected the practical expedient permitting the use of the amounts disclosed in its pension and other postretirement benefit plan note for the prior comparative periods as the estimation basis for applying the retrospective presentation requirements. Accordingly, for the three months ended December 31, 2017, the Company, Spire Missouri and Spire Alabama reclassified net benefit (income) costs of $(1.1), $0.2 and $(1.0), respectively, from “operation and maintenance” to “other income, net” or “other (expense) income, net.” The corresponding annual amounts to be reclassified for the years ended September 30, 2018, 2017 and 2016 are $14.4, $19.2 and $9.6 for the Company, respectively, $17.2, $17.2 and $11.2 for Spire Missouri, respectively, and $(2.0), $2.2 and $(1.6) for Spire Alabama, respectively. For Spire Missouri, Spire Alabama, and the Company’s other rate-regulated entities, all components of net benefit cost have historically been recovered from customers as a component of utility plant and will continue to be recovered in the same manner over the depreciable lives of the related plant assets; therefore, for those entities, the components that are no longer eligible to be capitalized as a component of plant under GAAP will be reported as regulatory assets. In February 2016, the FASB issued ASU No. 2016-02, Leases. The new standard requires lessees to recognize a right-of-use asset and lease liability for almost all lease contracts based on the present value of lease payments. There is an exemption for short-term leases. The ASU provides new guidelines for identifying and classifying a lease, and classification affects the pattern and income statement line item for the related expense. ASU Nos. 2018-01, 2018-10, 2018-11, and 2018-20, issued in January, July and December of 2018, amended several aspects of the new lease guidance, including providing an additional practical expedient, an additional transition method, and clarification of the related transition and accounting for land easements. The updates may be applied using a modified retrospective transition approach for leases existing at, or entered into after, the beginning of (1) the earliest comparative period presented in the financial statements or (2) the period of adoption. The ASUs are effective for fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. The Company, Spire Missouri and Spire Alabama are currently assessing the impacts of adopting these standards in the first quarter of fiscal 2020. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses: Measurement of Credit Losses on Financial Instruments, which was subsequently amended by ASU No. 2018-19 in November 2018. The standard introduces new guidance for the accounting for credit losses on instruments within its scope, including trade receivables. It is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years, and may be adopted a year earlier. The new guidance will be initially applied through a cumulative-effect adjustment to retained earnings as of the beginning of the period of adoption. The Company, Spire Missouri and Spire Alabama are currently assessing the timing and impacts of adopting this standard, which must be adopted by the first quarter of fiscal 2021. In August 2017, the FASB issued ASU No. 2017-12, Derivatives and Hedging: Targeted Improvements to Accounting for Hedging Activities. The amendments in this ASU more closely align the results of hedge accounting with risk management activities through changes to both the designation and measurement guidance for qualifying hedging relationships and the presentation of hedge results in the financial statements. They are effective for fiscal years beginning after December 15, 2018, and interim periods within those fiscal years, and early application is permitted. The Company, Spire Missouri and Spire Alabama are currently assessing the effects of this new guidance, as well as the timing of adoption. In August 2018, the FASB issued ASU No. 2018-15, Intangibles – Goodwill and Other – Internal-Use Software: Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract. Previous GAAP did not specifically address the accounting for implementation costs of a hosting arrangement that is a service contract. The amendments in this update clarify that accounting and align the accounting for implementation costs for hosting arrangements, regardless of whether they convey a license to the hosted software. The amendments are effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years. Early adoption is permitted, including adoption in any interim period. The amendments in this ASU may be applied either retrospectively or prospectively to all implementation costs incurred after the date of adoption. The Company, Spire Missouri and Spire Alabama are still assessing the effects of this new guidance but currently expect early adoption.

Summary of Significant Accounting Policies (Policies)

3 Months Ended

Dec. 31, 2018

BASIS OF PRESENTATION – These notes are an integral part of the accompanying unaudited financial statements of Spire Inc. presented on a consolidated basis (“Spire” or the “Company”), Spire Missouri Inc. (“Spire Missouri” or the “Missouri Utilities”) and Spire Alabama Inc. (“Spire Alabama”). Spire Missouri and Spire Alabama are wholly owned subsidiaries of the Company. Spire Missouri, Spire Alabama and the subsidiaries of Spire EnergySouth Inc. (“Spire EnergySouth”) are collectively referred to as the “Utilities.” The subsidiaries of Spire EnergySouth are Spire Gulf Inc. and Spire Mississippi Inc.

The accompanying unaudited financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) for interim financial information with the instructions to Form 10-Q and Rule 10-01 of Regulation S‑X. Accordingly, they do not include all the disclosures required for complete financial statements. In the opinion of management, the accompanying unaudited financial statements include all adjustments (consisting of only normal recurring adjustments) necessary for the fair presentation of the results of operations for the periods presented. This Form 10-Q should be read in conjunction with the Notes to Financial Statements contained in Spire’s, Spire Missouri’s and Spire Alabama’s combined Annual Report on Form 10-K for the fiscal year ended September 30, 2018.

The consolidated financial position, results of operations, and cash flows of Spire include the accounts of the Company and all its subsidiaries. Transactions and balances between consolidated entities have been eliminated from the consolidated financial statements of Spire. In compliance with GAAP, transactions between Spire Missouri and Spire Alabama and their affiliates, as well as intercompany balances on their balance sheets, have not been eliminated from their separate financial statements.

Derivatives

DERIVATIVES – In the course of their business, certain subsidiaries of the Company enter into commitments associated with the purchase or sale of natural gas. Certain of their derivative natural gas contracts are designated as normal purchases or normal sales and, as such, are excluded from the scope of Financial Accounting Standards Board (“FASB”) Accounting Standards Codification (“ASC”) Topic 815, Derivatives and Hedging. Those contracts are accounted for as executory contracts and recorded on an accrual basis. Revenues and expenses from such contracts are recorded using a gross presentation. Contracts not designated as normal purchases or normal sales are recorded as derivatives with changes in fair value recognized in earnings in the periods prior to physical delivery. Certain of Spire Marketing’s wholesale purchase and sale transactions are classified as trading activities for financial reporting purposes. Under GAAP, revenues and expenses associated with trading activities are presented on a net basis in Gas Marketing operating revenues (or expenses, if negative) in the Condensed Consolidated Statements of Income. This net presentation has no effect on operating income or net income.

Regulated Operations

REGULATED OPERATIONS – The Utilities account for their regulated operations in accordance with FASB ASC Topic 980, Regulated Operations. This topic sets forth the application of GAAP for those companies whose rates are established by or are subject to approval by an independent third-party regulator. The provisions of this accounting guidance require, among other things, that financial statements of a regulated enterprise reflect the actions of regulators, where appropriate. These actions may result in the recognition of revenues and expenses in time periods that are different than non-regulated enterprises. When this occurs, costs are deferred as assets in the balance sheet (regulatory assets) and recorded as expenses when those amounts are reflected in rates. In addition, regulators can impose liabilities upon a regulated company for amounts previously collected from customers and for recovery of costs that are expected to be incurred in the future (regulatory liabilities). Management believes that the current regulatory environment supports the continued use of these regulatory accounting principles and that all regulatory assets and regulatory liabilities are recoverable or refundable through the regulatory process.

As authorized by the Missouri Public Service Commission (“MoPSC”), the Mississippi Public Service Commission (“MSPSC”) and the Alabama Public Service Commission (“APSC”), the Purchased Gas Adjustment (“PGA”) clauses and Gas Supply Adjustment (“GSA”) riders allow the Utilities to pass through to customers the cost of purchased gas supplies. Regulatory assets and liabilities related to the PGA clauses and the GSA riders are both labeled Unamortized Purchased Gas Adjustments herein. See additional information about regulatory assets and liabilities in Note 4, Regulatory Matters.

Transactions with Affiliates

TRANSACTIONS WITH AFFILIATES – Transactions between affiliates of the Company have been eliminated from the consolidated statements of Spire. Spire Missouri and Spire Alabama borrowed funds from the Company and incurred related interest, as reflected in their separate financial statements, and they participated in normal intercompany shared services transactions

Accrued Capital Expenditures

Accrued capital expenditures, shown in the following table, are excluded from capital expenditures in the statements of cash flows until paid.

New Accounting Pronouncements

NEW ACCOUNTING PRONOUNCEMENTS – Spire, Spire Missouri and Spire Alabama adopted the guidance in Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers, and related amendments (collectively, “ASC 606”), in the first quarter of fiscal year 2019 using the modified retrospective method applied to all contracts at October 1, 2018. The core principle of ASC 606 is that revenue should be recognized to depict the transfer of control of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. Under the new standard, an entity must identify the performance obligations in a contract, determine the transaction price and allocate the price to specific performance obligations to recognize revenue when the obligation is completed. In addition, ASC 606 requires disclosure of the nature, amount, timing and uncertainty of revenue and cash flows arising from contracts with customers. The adoption of ASC 606 did not result in significant changes to how the Spire, Spire Missouri and Spire Alabama recognize revenue, and therefore, no cumulative effect adjustment to the opening balance of retained earnings was required, and there was no significant impact to financial results after adoption. The adoption did result in changes to the disclosures about revenue, which are included in Note 2, Revenue. Some revenue arrangements, such as alternative revenue programs and certain derivative contracts, are excluded from the scope of ASC 606 and, therefore, are presented separately in disclosures.

Also effective October 1, 2018, Spire, Spire Missouri and Spire Alabama adopted ASU No. 2017-07, Compensation – Retirement Benefits: Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost, with no material impact on interim or annual financial statements. The amended guidance requires that the service cost component of net periodic pension and postretirement benefit costs be presented within the same line item in the income statement as other compensation costs (except for the amount being capitalized), while other components are to be presented outside the subtotal of operating income and are no longer eligible for capitalization (e.g., as part of utility plant). The amended guidance is applied retrospectively for income statement presentation and prospectively for capitalization. The Company, Spire Missouri and Spire Alabama elected the practical expedient permitting the use of the amounts disclosed in its pension and other postretirement benefit plan note for the prior comparative periods as the estimation basis for applying the retrospective presentation requirements. Accordingly, for the three months ended December 31, 2017, the Company, Spire Missouri and Spire Alabama reclassified net benefit (income) costs of $(1.1), $0.2 and $(1.0), respectively, from “operation and maintenance” to “other income, net” or “other (expense) income, net.” The corresponding annual amounts to be reclassified for the years ended September 30, 2018, 2017 and 2016 are $14.4, $19.2 and $9.6 for the Company, respectively, $17.2, $17.2 and $11.2 for Spire Missouri, respectively, and $(2.0), $2.2 and $(1.6) for Spire Alabama, respectively. For Spire Missouri, Spire Alabama, and the Company’s other rate-regulated entities, all components of net benefit cost have historically been recovered from customers as a component of utility plant and will continue to be recovered in the same manner over the depreciable lives of the related plant assets; therefore, for those entities, the components that are no longer eligible to be capitalized as a component of plant under GAAP will be reported as regulatory assets.

In February 2016, the FASB issued ASU No. 2016-02, Leases. The new standard requires lessees to recognize a right-of-use asset and lease liability for almost all lease contracts based on the present value of lease payments. There is an exemption for short-term leases. The ASU provides new guidelines for identifying and classifying a lease, and classification affects the pattern and income statement line item for the related expense. ASU Nos. 2018-01, 2018-10, 2018-11, and 2018-20, issued in January, July and December of 2018, amended several aspects of the new lease guidance, including providing an additional practical expedient, an additional transition method, and clarification of the related transition and accounting for land easements. The updates may be applied using a modified retrospective transition approach for leases existing at, or entered into after, the beginning of (1) the earliest comparative period presented in the financial statements or (2) the period of adoption. The ASUs are effective for fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. The Company, Spire Missouri and Spire Alabama are currently assessing the impacts of adopting these standards in the first quarter of fiscal 2020.

In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses: Measurement of Credit Losses on Financial Instruments, which was subsequently amended by ASU No. 2018-19 in November 2018. The standard introduces new guidance for the accounting for credit losses on instruments within its scope, including trade receivables. It is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years, and may be adopted a year earlier. The new guidance will be initially applied through a cumulative-effect adjustment to retained earnings as of the beginning of the period of adoption. The Company, Spire Missouri and Spire Alabama are currently assessing the timing and impacts of adopting this standard, which must be adopted by the first quarter of fiscal 2021.

In August 2017, the FASB issued ASU No. 2017-12, Derivatives and Hedging: Targeted Improvements to Accounting for Hedging Activities. The amendments in this ASU more closely align the results of hedge accounting with risk management activities through changes to both the designation and measurement guidance for qualifying hedging relationships and the presentation of hedge results in the financial statements. They are effective for fiscal years beginning after December 15, 2018, and interim periods within those fiscal years, and early application is permitted. The Company, Spire Missouri and Spire Alabama are currently assessing the effects of this new guidance, as well as the timing of adoption.

In August 2018, the FASB issued ASU No. 2018-15, Intangibles – Goodwill and Other – Internal-Use Software: Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract. Previous GAAP did not specifically address the accounting for implementation costs of a hosting arrangement that is a service contract. The amendments in this update clarify that accounting and align the accounting for implementation costs for hosting arrangements, regardless of whether they convey a license to the hosted software. The amendments are effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years. Early adoption is permitted, including adoption in any interim period. The amendments in this ASU may be applied either retrospectively or prospectively to all implementation costs incurred after the date of adoption. The Company, Spire Missouri and Spire Alabama are still assessing the effects of this new guidance but currently expect early adoption.