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PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Narrative (Details) - USD ($) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2015 | Jan. 01, 2011 | Feb. 20, 2010
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Extent of Lump-sum payments recognized, percentage | 100.00% |  |  |  |
Implementation period used to calculate return on plan assets | 4 years |  |  |  |
Gains or losses includible in cost amortized to extent exceeds benefit obligations or market-related value of plan assets, percentage | 10.00% |  |  |  |
Anticipated Contributions to pension plans for qualified trust | $ 35.5 |  |  |  |
Anticipated contributions to pension plans for non-qualified plans | $ 0.5 |  |  |  |
Medical insurance available until age is reached after early retirement | 65 years |  |  |  |
Recovery in rates for postretirement benefit |  |  |  | $ 9.5 |
Spire Missouri |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Recovery in rates for the qualified pension plans base allowance |  |  |  | $ 15.5 |
Other Plans [Abstract] |  |  |  |  |
401(K) plans, cost of defined contributions | $ 8.4 | $ 8.2 | $ 8.0 |  |
Spire Alabama |  |  |  |  |
Other Plans [Abstract] |  |  |  |  |
401(K) plans, cost of defined contributions | 2.7 | 2.3 | 3.0 |  |
Spire Missouri West |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Recovery in rates for the qualified pension plans base allowance |  |  |  |  | $ 10.0
Pension Plans |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Settlement benefits paid | $ 62.2 | $ 16.6 | 71.1 |  |
Pension Plans | Spire Missouri |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Targeted allocation of plan assets | 100.00% | 100.00% |  |  |
Settlement benefits paid | $ 43.5 | $ 0.0 | 58.2 |  |
Pension Plans | Spire Alabama |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Targeted allocation of plan assets | 100.00% | 100.00% |  |  |
Settlement benefits paid | $ 18.7 | $ 16.6 | $ 12.9 |  |
Postretirement Plans |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Anticipated contributions to the postretirement plans for qualified trusts | 7.2 |  |  |  |
Anticipated payment directly to participants in postretirement plan | $ 0.2 |  |  |  |
Postretirement Plans | Spire Missouri |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Targeted allocation of plan assets | 100.00% |  |  |  |
Postretirement Plans | Spire Alabama |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Targeted allocation of plan assets | 100.00% |  |  |  |
Equity markets | Pension Plans | Spire Missouri |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Targeted allocation of plan assets | 56.40% | 56.20% |  |  |
Equity markets | Pension Plans | Spire Alabama |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Targeted allocation of plan assets | 60.00% | 60.00% |  |  |
Equity markets | Postretirement Plans | Spire Missouri |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Targeted allocation of plan assets | 60.00% |  |  |  |
Equity markets | Postretirement Plans | Spire Alabama |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Targeted allocation of plan assets | 60.00% |  |  |  |
Debt securities | Pension Plans | Spire Missouri |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Targeted allocation of plan assets | 43.60% | 43.80% |  |  |
Debt securities | Postretirement Plans | Spire Missouri |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Targeted allocation of plan assets | 40.00% |  |  |  |
Debt securities | Postretirement Plans | Spire Alabama |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Targeted allocation of plan assets | 40.00% |  |  |  |

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- Definition Anticipated contributions to the pension plans for fiscal year 2012 for non-qualified plans. + References No definition available. + Details
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- Definition Anticipated contributions to the pension plans for the next fiscal year for qualified trusts. + References No definition available. + Details
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- Definition Anticipated contribution to the postretirement plans for the next fiscal year for qualified trusts. + References No definition available. + Details
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- Definition Defined Benefit Plan, Lump-sum Payment Recognized as Settlement Gain (Loss), Minimum Threshold + References No definition available. + Details
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- Definition Gains and losses not yet includible in pension/postretirement benefit cost are amortized only to the extent that such gain or loss exceeds 10% of the greater of the benefit obligation or the market-related value of plan assets + References No definition available. + Details
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- Definition Disclosure under Missouri Public Service Commission Order, the return on plan assets if based on the market-related value of plan assets implemented prospectively over a four-year period. + References No definition available. + Details
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- Definition The amount of annual recovery of postretirement benefit costs allowed in rates by the Missouri Public Service Commission. The differences between these amounts and those calculated pursuant to GAAP are deferred as regulatory assets or liabilities. + References No definition available. + Details
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- Definition Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(5)(i) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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