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PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Amounts Recognized in Consolidated Balance Sheets (Details) - USD ($) $ in Millions | Sep. 30, 2017 | Sep. 30, 2016
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Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] |  |
Noncurrent liabilities | $ (237.4) | $ (303.7)
Pension Plans |  |
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] |  |
Current liabilities | (0.5) | (0.6)
Noncurrent liabilities | (216.7) | (253.6)
Total | (217.2) | (254.2)
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of: |  |
Net actuarial loss | 163.0 | 179.4
Prior service (credit) cost | (13.4) | 8.2
Subtotal | 149.6 | 187.6
Adjustments for amounts included in regulatory assets | (147.1) | (184.8)
Total | 2.5 | 2.8
Postretirement Plans |  |
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] |  |
Current liabilities | (0.4) | (0.4)
Noncurrent liabilities | (21.0) | (50.1)
Total | 27.0 | (12.8)
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of: |  |
Net actuarial loss | 1.5 | 38.0
Prior service (credit) cost | (6.6) | (5.2)
Subtotal | (5.1) | 32.8
Adjustments for amounts included in regulatory assets | 5.1 | (32.8)
Total | 0.0 | 0.0
Spire Missouri |  |
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] |  |
Noncurrent liabilities | (173.0) | (211.8)
Spire Missouri | Pension Plans |  |
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] |  |
Current liabilities | (0.5) | (0.6)
Noncurrent liabilities | (153.2) | (163.7)
Total | (153.7) | (164.3)
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of: |  |
Net actuarial loss | 126.2 | 135.5
Prior service (credit) cost | 7.3 | 8.2
Subtotal | 133.5 | 143.7
Adjustments for amounts included in regulatory assets | (131.0) | (140.9)
Total | 2.5 | 2.8
Spire Missouri | Postretirement Plans |  |
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] |  |
Current liabilities | (0.4) | (0.4)
Noncurrent liabilities | (19.8) | (48.1)
Total | (17.6) | (48.2)
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of: |  |
Net actuarial loss | 12.3 | 43.4
Prior service (credit) cost | (3.7) | (3.4)
Subtotal | 8.6 | 40.0
Adjustments for amounts included in regulatory assets | (8.6) | (40.0)
Total | 0.0 | 0.0
Spire Alabama |  |
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] |  |
Noncurrent liabilities | (50.2) | (74.3)
Spire Alabama | Pension Plans |  |
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] |  |
Current liabilities | 0.0 | 0.0
Noncurrent liabilities | (50.3) | (74.3)
Total | (50.3) | (74.3)
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of: |  |
Net actuarial loss | 40.9 | 43.9
Prior service (credit) cost | (20.7) | 0.0
Subtotal | 20.2 | 43.9
Adjustments for amounts included in regulatory assets | (20.2) | (43.9)
Total | 0.0 | 0.0
Spire Alabama | Postretirement Plans |  |
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] |  |
Current liabilities | 0.0 | 0.0
Noncurrent liabilities | 0.0 | 0.0
Total | 45.8 | 37.4
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of: |  |
Net actuarial loss | (9.7) | (5.4)
Prior service (credit) cost | (2.9) | (1.8)
Subtotal | (12.6) | (7.2)
Adjustments for amounts included in regulatory assets | 12.6 | 7.2
Total | 0.0 | 0.0
Current Asset [Member] | Postretirement Plans |  |
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] |  |
Noncurrent assets | 1.4 | 0.3
Current Asset [Member] | Spire Missouri | Postretirement Plans |  |
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] |  |
Noncurrent assets | 1.4 | 0.3
Current Asset [Member] | Spire Alabama | Postretirement Plans |  |
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] |  |
Noncurrent assets | 0.0 | 0.0
Noncurrent Asset [Member] | Postretirement Plans |  |
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] |  |
Noncurrent assets | 47.0 | 37.4
Noncurrent Asset [Member] | Spire Missouri | Postretirement Plans |  |
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] |  |
Noncurrent assets | 1.2 | 0.0
Noncurrent Asset [Member] | Spire Alabama | Postretirement Plans |  |
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] |  |
Noncurrent assets | $ 45.8 | $ 37.4

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