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Summary of Significant Accounting Policies - Additional Information (Details) | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Oct. 31, 2018 USD ($) | May 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Jun. 30, 2020 USD ($) Station | Sep. 30, 2020 USD ($) | Sep. 30, 2020 USD ($) | Sep. 30, 2020 USD ($) ReportingUnit | Sep. 30, 2020 USD ($) | Sep. 30, 2020 USD ($) operating-segment | Sep. 30, 2019 USD ($) | Sep. 30, 2018 USD ($) | Dec. 31, 2020 USD ($) | Oct. 25, 2018
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of reportable segments |  |  |  |  |  |  | 2 |  | 2 |  |  |  |
Revisions in estimated cash flows |  |  |  |  |  | $ 192,400,000 |  |  |  | $ 1,000,000.0 |  |  |
Difference between replacement cost of gas storage and inventory basis amount |  |  |  |  |  | 12,500,000 |  |  |  | 17,000,000.0 |  |  |
Goodwill |  |  |  |  | $ 1,171,600,000 | 1,171,600,000 | $ 1,171,600,000 | $ 1,171,600,000 | $ 1,171,600,000 | 1,171,600,000 |  |  |
Goodwill accumulated impairment loss |  |  |  |  | $ 0 | 0 | 0 | 0 | 0 |  |  |  |
Impairment charge of long-lived assets |  |  |  |  |  | 148,600,000 |  |  |  | $ 0 | $ 0 |  |
Number of months of the original maturity of debt instruments purchased |  |  |  |  | 3 months |  |  |  |  |  |  |  |
Restricted cash |  |  |  |  |  |  |  |  |  |  | 17,000,000.0 |  |
ASU 2017-07 |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Reclassification of net periodic benefit costs (income) to other income (expense) |  |  |  |  |  |  |  |  |  |  | $ 14,400,000 |  |
ASU 2016-13 | Scenario Forecast |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Credit loss allowances for available-for-sale securities recognized at adoption of ASU |  |  |  |  |  |  |  |  |  |  |  | $ 0 |
Gas Utility |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Goodwill |  |  |  |  | $ 210,200,000 | 210,200,000 | 210,200,000 | 210,200,000 | 210,200,000 |  |  |  |
Gas Marketing |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Goodwill |  |  |  |  | $ 0 | $ 0 | $ 0 | $ 0 | $ 0 |  |  |  |
Natural Gas Storage Facility |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of voting interest acquired | 20.00% | 100.00% | 80.00% |  |  |  |  |  |  |  |  |  |
Consideration | $ 17,000,000.0 | $ 12,200,000 | $ 24,800,000 |  |  |  |  |  |  |  |  |  |
Payments to acquire business |  |  | 16,000,000.0 |  |  |  |  |  |  |  |  |  |
Non-interest-bearing note |  |  | 10,000,000.0 |  |  |  |  |  |  |  |  |  |
Non-interest-bearing note, value |  |  | $ 8,800,000 |  |  |  |  |  |  |  |  |  |
Percentage of voting interest |  |  |  |  |  |  |  |  |  |  |  |  | 20.00%
Spire Missouri |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Average annual depreciation and amortization expense percentage |  |  |  |  | 2.90% | 2.90% | 2.90% | 2.90% | 2.90% | 3.00% | 3.00% |  |
Revisions in estimated cash flows |  |  |  |  |  | $ (25,900,000) |  |  |  | $ (2,800,000) |  |  |
Goodwill |  |  |  |  | $ 210,200,000 | 210,200,000 | $ 210,200,000 | $ 210,200,000 | $ 210,200,000 | 210,200,000 |  |  |
Spire Missouri | ASU 2017-07 |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Reclassification of net periodic benefit costs (income) to other income (expense) |  |  |  |  |  |  |  |  |  |  | $ 17,200,000 |  |
Spire Missouri | ASU 2016-13 | Scenario Forecast |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Credit loss allowances for available-for-sale securities recognized at adoption of ASU |  |  |  |  |  |  |  |  |  |  |  | 0 |
Spire Missouri | Affiliated Entity | Spire Marketing | Regulated Operation |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Purchases of natural gas from related party |  |  |  |  |  | 56,900,000 |  |  |  | 95,300,000 | 71,500,000 |  |
Sales of natural gas to Spire Marketing Inc. |  |  |  |  |  | 100,000 |  |  |  | $ 1,700,000 | $ 300,000 |  |
Spire Alabama |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Composite depreciation rate |  |  |  |  |  |  |  | 3.10% |  | 3.10% | 3.10% |  |
Revisions in estimated cash flows |  |  |  |  |  | 221,100,000 |  |  |  | $ 0 |  |  |
Finance receivable, net |  |  |  |  | $ 9,400,000 | $ 9,400,000 | $ 9,400,000 | $ 9,400,000 | $ 9,400,000 | 11,700,000 |  |  |
Threshold period past due for collection |  |  |  |  | 90 days |  |  |  |  |  |  |  |
Threshold period past due for write-off of financing receivable |  |  |  |  | 12 months | 12 months | 12 months | 12 months | 12 months |  |  |  |
Spire Alabama | ASU 2017-07 |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Reclassification of net periodic benefit costs (income) to other income (expense) |  |  |  |  |  |  |  |  |  |  | $ (2,000,000.0) |  |
Spire Alabama | ASU 2016-13 | Scenario Forecast |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Credit loss allowances for available-for-sale securities recognized at adoption of ASU |  |  |  |  |  |  |  |  |  |  |  | $ 0 |
Spire Alabama | Financing Receivables, Equal to Greater than 90 Days Past Due |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Finance receivable 90 days past due |  |  |  |  | $ 300,000 | $ 300,000 | $ 300,000 | $ 300,000 | $ 300,000 | $ 400,000 |  |  |
Spire Alabama | Affiliated Entity | Spire Marketing | Regulated Operation |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Purchases of natural gas from related party |  |  |  |  |  | 6,300,000 |  |  |  |  |  |  |
Sales of natural gas to Spire Marketing Inc. |  |  |  |  |  | $ 300,000 |  |  |  |  |  |  |
Spire Storage West LLC ("Spire Storage") | Non-Utility Property |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Impairment charge of long-lived assets |  |  |  | $ 140,800,000 |  |  |  |  |  |  |  |  |
Number of commercial stations impaired | Station |  |  |  | 2 |  |  |  |  |  |  |  |  |
Spire Storage West LLC ("Spire Storage") | Two Compressed Natural Gas Fueling Stations (Non-utility Property) |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Impairment charge of long-lived assets |  |  |  | $ 7,800,000 |  |  |  |  |  |  |  |  |

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