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PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Net Periodic Cost (Details) - USD ($) $ in Millions | 3 Months Ended
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Dec. 31, 2016 | Dec. 31, 2015
Pension Plans |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Service cost – benefits earned during the period | $ 5.3 | $ 3.9
Interest cost on projected benefit obligation | 6.9 | 7.1
Expected return on plan assets | (9.9) | (8.9)
Amortization of prior service cost (credit) | 0.2 | 0.1
Amortization of actuarial gain (loss) | 3.4 | 2.0
Special termination benefits | 0.0 | 1.6
Subtotal | 5.9 | 5.8
Regulatory adjustment | 4.6 | 5.0
Net pension cost (income) | 10.5 | 10.8
Postretirement Plans |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Service cost – benefits earned during the period | 2.8 | 2.8
Interest cost on projected benefit obligation | 2.1 | 2.5
Expected return on plan assets | (3.4) | (3.4)
Amortization of prior service cost (credit) | 0.0 | 0.1
Amortization of actuarial gain (loss) | 0.6 | 0.9
Special termination benefits | 0.0 | 2.6
Subtotal | 2.1 | 5.5
Regulatory adjustment | (0.8) | (4.2)
Net pension cost (income) | 1.3 | 1.3
Laclede Gas | Pension Plans |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Service cost – benefits earned during the period | 3.3 | 2.5
Interest cost on projected benefit obligation | 4.8 | 5.4
Expected return on plan assets | (7.3) | (6.7)
Amortization of prior service cost (credit) | 0.2 | 0.1
Amortization of actuarial gain (loss) | 2.9 | 2.0
Special termination benefits | 0.0 | 1.6
Subtotal | 3.9 | 4.9
Regulatory adjustment | 2.8 | 3.5
Net pension cost (income) | 6.7 | 8.4
Laclede Gas | Postretirement Plans |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Service cost – benefits earned during the period | 2.6 | 2.7
Interest cost on projected benefit obligation | 1.7 | 2.0
Expected return on plan assets | (2.3) | (2.1)
Amortization of prior service cost (credit) | 0.1 | 0.1
Amortization of actuarial gain (loss) | 0.6 | 1.0
Special termination benefits | 0.0 | 2.6
Subtotal | 2.7 | 6.3
Regulatory adjustment | (0.4) | (3.8)
Net pension cost (income) | 2.3 | 2.5
Alagasco | Pension Plans |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Service cost – benefits earned during the period | 1.6 | 1.4
Interest cost on projected benefit obligation | 1.5 | 1.7
Expected return on plan assets | (1.8) | (2.2)
Amortization of actuarial gain (loss) | 0.5 | 0.0
Subtotal | 1.8 | 0.9
Regulatory adjustment | 1.6 | 1.5
Net pension cost (income) | 3.4 | 2.4
Alagasco | Postretirement Plans |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Service cost – benefits earned during the period | 0.1 | 0.1
Interest cost on projected benefit obligation | 0.4 | 0.5
Expected return on plan assets | (1.1) | (1.3)
Amortization of prior service cost (credit) | (0.1) | 0.0
Amortization of actuarial gain (loss) | 0.0 | (0.1)
Subtotal | (0.7) | (0.8)
Regulatory adjustment | (0.4) | (0.4)
Net pension cost (income) | $ (1.1) | $ (1.2)

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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 The amount of gains or losses recognized in net periodic benefit cost. + 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=65897934&loc=d3e1928-114920 + Details
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- Definition The amount of the prior service cost or credit recognized in net periodic benefit cost relating to benefit changes attributable to plan participants' prior service pursuant to a plan amendment or a plan initiation. + 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=65897934&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Prior Service Cost -URI http://asc.fasb.org/extlink&oid=49177896 + Details
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- Definition If applicable, the cost of providing special or contractual termination benefits recognized during the period on the projected benefit obligation. + 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=65897934&loc=d3e1928-114920 + Details
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- Definition An amount calculated as a basis for determining the extent of delayed recognition of the effects of changes in the fair value of assets. The expected return on plan assets is determined based on the expected long-term rate of return on plan assets and the market-related value of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Expected Return on Plan Assets -URI http://asc.fasb.org/extlink&oid=6512171 Reference 2: 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=65897934&loc=d3e1928-114920 + Details
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- Definition The increase in a defined benefit pension plan's projected benefit obligation or a defined benefit postretirement plan's accumulated postretirement benefit obligation due to the passage of time. + 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=65897934&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=65897934&loc=d3e1928-114920 + Details
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- Definition The total amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments. + 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=65897934&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 (j) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition The actuarial present value of benefits attributed by the pension benefit formula to services rendered by employees during the period. The portion of the expected postretirement benefit obligation attributed to employee service during the period. The service cost component is a portion of the benefit obligation and is unaffected by the funded status of the plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Service Cost (Component of Net Periodic Pension Cost) -URI http://asc.fasb.org/extlink&oid=6525008 Reference 2: 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=65897934&loc=d3e1928-114920 Reference 3: 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=65897934&loc=d3e1928-114920 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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