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COMMITMENTS AND CONTINGENCIES - Leases and Guarantees (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2015
Loss Contingencies [Line Items] |  |  |
Aggregate rental expense | $ 9.7 | $ 11.9 | $ 14.1
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract] |  |  |
Minimum rental commitments - in fiscal year 2017 | 10.1 |  |
Minimum rental commitments - due in fiscal year 2018 | 9.3 |  |
Minimum rental commitments - due in fiscal year 2019 | 7.8 |  |
Minimum rental commitments - due in fiscal year 2020 | 6.1 |  |
Minimum rental commitments - due in fiscal year 2021 | 6.0 |  |
Minimum rental commitments - due later | 44.4 |  |
Minimum rental commitments - total | 83.7 |  |
Sublease rentals | 2.1 | 2.1 | 0.2
Minimum future sublease rentals to be received - fiscal year 2017 | 2.1 |  |
Minimum future sublease rentals to be received - fiscal year 2018 | 2.1 |  |
Minimum future sublease rentals to be received - fiscal year 2019 | 2.1 |  |
Minimum future sublease rentals to be received - fiscal year 2020 | 0.5 |  |
Outstanding mortgage loans incurred in connection with various real estate ventures with general partners | 1.3 |  |
Spire Missouri |  |  |
Loss Contingencies [Line Items] |  |  |
Aggregate rental expense | 4.8 | 4.3 | 7.6
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract] |  |  |
Minimum rental commitments - in fiscal year 2017 | 2.1 |  |
Minimum rental commitments - due in fiscal year 2018 | 1.3 |  |
Minimum rental commitments - due in fiscal year 2019 | 0.2 |  |
Minimum rental commitments - due in fiscal year 2020 | 0.2 |  |
Minimum rental commitments - due in fiscal year 2021 | 0.0 |  |
Minimum rental commitments - due later | 0.0 |  |
Minimum rental commitments - total | 3.8 |  |
Spire Alabama |  |  |
Loss Contingencies [Line Items] |  |  |
Aggregate rental expense | 4.6 | 3.7 | 4.0
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract] |  |  |
Minimum rental commitments - in fiscal year 2017 | 4.0 |  |
Minimum rental commitments - due in fiscal year 2018 | 4.1 |  |
Minimum rental commitments - due in fiscal year 2019 | 3.8 |  |
Minimum rental commitments - due in fiscal year 2020 | 2.1 |  |
Minimum rental commitments - due in fiscal year 2021 | 2.1 |  |
Minimum rental commitments - due later | 2.8 |  |
Minimum rental commitments - total | 18.9 |  |
Sublease rentals | 2.1 | $ 2.1 | $ 2.1
Minimum future sublease rentals to be received - fiscal year 2018 | 2.1 |  |
Minimum future sublease rentals to be received - fiscal year 2019 | 2.1 |  |
Minimum future sublease rentals to be received - fiscal year 2020 | $ 0.5 |  |

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- Definition Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 10 -Section 55 -Paragraph 40 -Subparagraph (Note 3) -URI http://asc.fasb.org/extlink&oid=82846649&loc=d3e38371-112697 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=77902758&loc=d3e41502-112717 + Details
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