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PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Fair Value of Plan Assets (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2015
Pension Plans |  |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |  |
Fair value of plan assets, beginning of year | $ 540.5 | $ 448.9 |
Actual return on plan assets | 38.0 | 75.1 |
Employer contributions | 41.3 | 26.6 |
Spire EnergySouth acquisition | 0.0 | 44.8 |
Settlement benefits paid | (62.2) | (16.6) | $ (71.1)
Regular benefits paid | (26.0) | (38.3) |
Fair value of plan assets, end of year | 531.6 | 540.5 | 448.9
Funded status of plans, end of year | (217.2) | (254.2) |
Postretirement Plans |  |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |  |
Fair value of plan assets, beginning of year | 246.4 | 223.3 |
Actual return on plan assets | 26.2 | 19.9 |
Employer contributions | 10.4 | 14.3 |
Spire EnergySouth acquisition | 0.0 | 4.0 |
Regular benefits paid | (17.5) | (15.1) |
Fair value of plan assets, end of year | 265.5 | 246.4 | 223.3
Funded status of plans, end of year | 27.0 | (12.8) |
Spire Missouri | Pension Plans |  |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |  |
Fair value of plan assets, beginning of year | 395.7 | 339.9 |
Actual return on plan assets | 25.1 | 64.4 |
Employer contributions | 29.4 | 26.6 |
Spire EnergySouth acquisition | 0.0 | 0.0 |
Settlement benefits paid | (43.5) | 0.0 | (58.2)
Regular benefits paid | (20.8) | (35.2) |
Fair value of plan assets, end of year | 385.9 | 395.7 | 339.9
Funded status of plans, end of year | (153.7) | (164.3) |
Spire Missouri | Postretirement Plans |  |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |  |
Fair value of plan assets, beginning of year | 159.7 | 143.6 |
Actual return on plan assets | 16.8 | 13.8 |
Employer contributions | 10.4 | 14.3 |
Spire EnergySouth acquisition | 0.0 | 0.0 |
Regular benefits paid | (12.0) | (12.0) |
Fair value of plan assets, end of year | 174.9 | 159.7 | 143.6
Funded status of plans, end of year | (17.6) | (48.2) |
Spire Alabama | Pension Plans |  |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |  |
Fair value of plan assets, beginning of year | 100.0 | 109.0 |
Actual return on plan assets | 7.7 | 10.7 |
Employer contributions | 11.9 | 0.0 |
Spire EnergySouth acquisition | 0.0 | 0.0 |
Settlement benefits paid | (18.7) | (16.6) | (12.9)
Regular benefits paid | (3.0) | (3.1) |
Fair value of plan assets, end of year | 97.9 | 100.0 | 109.0
Funded status of plans, end of year | (50.3) | (74.3) |
Spire Alabama | Postretirement Plans |  |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |  |
Fair value of plan assets, beginning of year | 82.8 | 79.7 |
Actual return on plan assets | 8.9 | 6.2 |
Employer contributions | 0.0 | 0.0 |
Spire EnergySouth acquisition | 0.0 | 0.0 |
Regular benefits paid | (5.3) | (3.1) |
Fair value of plan assets, end of year | 86.4 | 82.8 | $ 79.7
Funded status of plans, end of year | $ 45.8 | $ 37.4 |

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- Definition Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses. + 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 (b)(1) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of (increase) decrease to benefit obligation of defined benefit plan from remeasurement due to settlement. Excludes decrease to benefit obligation for settlement payment. + 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 (a)(10) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of increase in plan assets of defined benefit plan from business combination. + 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 (b)(6) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of contribution received by defined benefit plan from employer which increases plan assets. + 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)(ii) -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 6 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2709-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(5)(iv)(01) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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