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Pension Plans and Other Postretirement Benefits - Schedule of Changes in Plan Assets and Benefit Obligations (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Amortization of actuarial gain (loss) | $ (0.5) | $ (0.9) | $ 0.4
Pension Plans |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Current year actuarial loss (gain) | 68.0 | 90.9 | (1.4)
Amortization of actuarial gain (loss) | (14.4) | (9.3) | (10.9)
Acceleration of loss recognized due to settlement | (31.7) | 0.0 | (18.5)
Current year service credit | (4.4) | (10.2) | (0.1)
Transfer due to merger | 0.0 | 0.0 | 0.1
Amortization of prior service credit (cost) | 2.5 | 1.1 | (2.9)
Amortization of transition asset | 0.0 | 0.0 | 1.8
Subtotal | 20.0 | 72.5 | (31.9)
Regulatory adjustment | (19.5) | (71.7) | 31.6
Total recognized in OCI | 0.5 | 0.8 | (0.3)
Other Postretirement Plans |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Current year actuarial loss (gain) | (7.3) | (12.2) | (45.1)
Amortization of actuarial gain (loss) | 2.0 | 0.5 | (0.8)
Current year prior service credit (cost) | 15.8 | 5.5 | 0.0
Amortization of current year service cost | 0.0 | 0.4 | 0.0
Amortization of prior service credit (cost) | 0.5 | (0.3) | 0.1
Subtotal | 11.0 | (6.1) | (45.8)
Regulatory adjustment | (11.0) | 6.1 | 45.8
Total recognized in OCI | 0.0 | 0.0 | 0.0
Spire Missouri | Pension Plans |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Current year actuarial loss (gain) | 37.8 | 59.0 | 2.2
Amortization of actuarial gain (loss) | (11.3) | (8.7) | (9.4)
Acceleration of loss recognized due to settlement | (26.6) | 0.0 | (16.1)
Current year service credit | (4.4) | (3.7) | 0.0
Transfer due to merger | 0.0 | 0.0 | 0.1
Amortization of prior service credit (cost) | (0.1) | (0.9) | (0.9)
Amortization of transition asset | 0.0 | 0.0 | 0.0
Subtotal | (4.6) | 45.7 | (24.1)
Regulatory adjustment | 5.1 | (44.9) | 23.8
Total recognized in OCI | 0.5 | 0.8 | (0.3)
Spire Missouri | Other Postretirement Plans |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Current year actuarial loss (gain) | (7.6) | (17.3) | (47.1)
Amortization of actuarial gain (loss) | 2.0 | 0.5 | (0.9)
Current year prior service credit (cost) | 9.5 | 4.9 | 0.0
Amortization of current year service cost | 0.0 | 0.0 | 0.0
Amortization of prior service credit (cost) | 0.2 | (0.3) | (0.3)
Subtotal | 4.1 | (12.2) | (48.3)
Regulatory adjustment | (4.1) | 12.2 | 48.3
Total recognized in OCI | 0.0 | 0.0 | 0.0
Spire Alabama | Pension Plans |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Current year actuarial loss (gain) | 24.4 | 26.3 | (0.6)
Amortization of actuarial gain (loss) | (3.1) | (0.8) | (1.5)
Acceleration of loss recognized due to settlement | (5.1) | 0.0 | (2.4)
Current year service credit | 0.0 | (6.5) | 0.0
Transfer due to merger | 0.0 | 0.0 | 0.0
Amortization of prior service credit (cost) | 2.4 | 1.8 | 0.0
Amortization of transition asset | 0.0 | 0.0 | 1.8
Subtotal | 18.6 | 20.8 | (2.7)
Regulatory adjustment | (18.6) | (20.8) | 2.7
Total recognized in OCI | 0.0 | 0.0 | 0.0
Spire Alabama | Other Postretirement Plans |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Current year actuarial loss (gain) | 1.1 | 5.5 | 1.6
Amortization of actuarial gain (loss) | 0.0 | 0.0 | 0.1
Current year prior service credit (cost) | 6.3 | 0.6 | 0.0
Amortization of current year service cost | 0.0 | 0.4 | 0.0
Amortization of prior service credit (cost) | 0.3 | 0.0 | 0.4
Subtotal | 7.7 | 6.5 | 2.1
Regulatory adjustment | (7.7) | (6.5) | (2.1)
Total recognized in OCI | $ 0.0 | $ 0.0 | $ 0.0

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