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PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Net Periodic Cost (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2015
Pension Plans |  |  |
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] |  |  |
Service cost – benefits earned during the period | $ 20.5 | $ 15.3 | $ 17.3
Interest cost on projected benefit obligation | 27.9 | 28.0 | 29.5
Expected return on plan assets | (38.5) | (34.9) | (37.4)
Amortization of prior service cost | 1.0 | 0.4 | 0.5
Amortization of actuarial loss | 12.5 | 8.0 | 7.5
Loss on lump-sum settlements and curtailments | 17.9 | 3.3 | 19.6
Special termination benefits | 0.9 | 1.6 | 0.0
Subtotal | 42.2 | 21.7 | 37.0
Regulatory adjustment | (2.4) | 17.8 | (2.1)
Net benefit cost | 39.8 | 39.5 | 34.9
Postretirement Plans |  |  |
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] |  |  |
Service cost – benefits earned during the period | 11.0 | 10.9 | 12.8
Interest cost on projected benefit obligation | 8.6 | 10.2 | 11.2
Expected return on plan assets | (13.6) | (13.5) | (13.2)
Amortization of prior service cost | 0.0 | 0.3 | 0.8
Amortization of actuarial loss | 2.5 | 3.6 | 5.1
Special termination benefits | 0.0 | 2.6 | 0.0
Subtotal | 8.5 | 14.1 | 16.7
Regulatory adjustment | (3.2) | (6.6) | (11.0)
Net benefit cost | 5.3 | 7.5 | 5.7
Spire Missouri | Pension Plans |  |  |
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] |  |  |
Service cost – benefits earned during the period | 12.7 | 10.0 | 11.5
Interest cost on projected benefit obligation | 19.5 | 21.7 | 23.3
Expected return on plan assets | (28.1) | (26.7) | (29.2)
Amortization of prior service cost | 1.0 | 0.4 | 0.5
Amortization of actuarial loss | 10.7 | 7.9 | 7.5
Loss on lump-sum settlements and curtailments | 13.5 | 0.0 | 18.0
Special termination benefits | 0.0 | 1.6 | 0.0
Subtotal | 29.3 | 14.9 | 31.6
Regulatory adjustment | (4.1) | 11.7 | (5.2)
Net benefit cost | 25.2 | 26.6 | 26.4
Spire Missouri | Postretirement Plans |  |  |
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] |  |  |
Service cost – benefits earned during the period | 10.4 | 10.6 | 12.3
Interest cost on projected benefit obligation | 6.8 | 8.1 | 8.6
Expected return on plan assets | (9.0) | (8.5) | (8.1)
Amortization of prior service cost | 0.2 | 0.3 | 0.8
Amortization of actuarial loss | 2.6 | 3.8 | 5.1
Special termination benefits | 0.0 | 2.6 | 0.0
Subtotal | 11.0 | 16.9 | 18.7
Regulatory adjustment | (1.5) | (4.8) | (9.2)
Net benefit cost | 9.5 | 12.1 | 9.5
Spire Alabama | Pension Plans |  |  |
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] |  |  |
Service cost – benefits earned during the period | 6.2 | 5.3 | 5.8
Interest cost on projected benefit obligation | 6.1 | 6.3 | 6.2
Expected return on plan assets | (7.2) | (8.2) | (8.2)
Amortization of prior service cost | 0.0 | 0.0 | 0.0
Amortization of actuarial loss | 1.8 | 0.1 | 0.0
Loss on lump-sum settlements and curtailments | 4.6 | 3.3 | 1.6
Special termination benefits | 0.0 | 0.0 | 0.0
Subtotal | 11.5 | 6.8 | 5.4
Regulatory adjustment | 1.8 | 6.1 | 3.1
Net benefit cost | 13.3 | 12.9 | 8.5
Spire Alabama | Postretirement Plans |  |  |
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] |  |  |
Service cost – benefits earned during the period | 0.3 | 0.3 | 0.5
Interest cost on projected benefit obligation | 1.6 | 2.1 | 2.6
Expected return on plan assets | (4.4) | (5.0) | (5.1)
Amortization of prior service cost | (0.2) | 0.0 | 0.0
Amortization of actuarial loss | (0.1) | (0.2) | 0.0
Special termination benefits | 0.0 | 0.0 | 0.0
Subtotal | (2.8) | (2.8) | (2.0)
Regulatory adjustment | (1.8) | (1.8) | (1.8)
Net benefit cost | $ (4.6) | $ (4.6) | $ (3.8)

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- Definition The total amount of periodic benefit cost for defined benefit plans for the period after the impact of regulatory adjustment. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain or loss, prior service cost or credit, transition asset or obligation, and gain or loss due to settlements or curtailments. + References No definition available. + Details
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- Definition The difference between the period's net benefit cost recognized in other comprehensive income for defined benefit plans calculated in accordance with GAAP and the amount of recovery allowed in rates by the Missouri Public Service Commission. These differences are deferred as regulatory assets or regulatory liabilities. + References No definition available. + Details
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- Definition Amount of gain (loss) recognized in net periodic benefit (cost) credit of defined benefit 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 (h)(4) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of prior service cost (credit) recognized in net periodic benefit cost (credit) of defined benefit 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 (h)(5) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of cost of providing special or contractual termination benefits payable from defined benefit 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 (q) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of expected return (loss) recognized in net periodic benefit (cost) credit, calculated based on expected long-term rate of return and market-related value of plan assets of defined benefit 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 (h)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of cost recognized for passage of time related to defined benefit 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 (h)(2) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of net periodic benefit cost (credit) for defined benefit 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 (h) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of gain (loss) recognized in net periodic benefit (cost) credit from irrevocable action relieving primary responsibility for benefit obligation and eliminating risk related to obligation and assets used to effect settlement. + 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 (h)(7) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of cost for actuarial present value of benefits attributed to service rendered by employee for defined benefit 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 (h)(1) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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