Pension Plans and Other Postretirement Benefits - Amounts Recognized in Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent liabilities	$ (264.8)	$ (180.2)
Spire Missouri
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent liabilities	(192.4)	(136.9)
Spire Alabama
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent liabilities	(59.2)	(35.0)
Pension Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Current liabilities	(0.5)	(0.6)
Noncurrent liabilities	(229.1)	(164.8)
Total	(229.6)	(165.4)
Pension Plans \| Spire Missouri
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Current liabilities	(0.5)	(0.6)
Noncurrent liabilities	(158.7)	(123.6)
Total	(159.2)	(124.2)
Pension Plans \| Spire Alabama
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Current liabilities	0.0	0.0
Noncurrent liabilities	(59.2)	(35.0)
Total	(59.2)	(35.0)
Other Postretirement Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Current liabilities	(0.5)	(0.5)
Noncurrent liabilities	(35.3)	(16.1)
Total	82.5	75.4
Other Postretirement Plans \| Noncurrent Asset [Member]
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent assets	118.3	92.0
Other Postretirement Plans \| Spire Missouri
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Current liabilities	(0.5)	(0.5)
Noncurrent liabilities	(33.8)	(14.3)
Total	41.0	30.7
Other Postretirement Plans \| Spire Missouri \| Noncurrent Asset [Member]
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent assets	75.3	45.5
Other Postretirement Plans \| Spire Alabama
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	43.0	46.5
Other Postretirement Plans \| Spire Alabama \| Noncurrent Asset [Member]
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent assets	$ 43.0	$ 46.5

Amount of asset (liability), recognized in statement of financial position, for defined benefit pension and other postretirement plans.

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Amount of asset, recognized in statement of financial position, for overfunded defined benefit pension and other postretirement plans.

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Amount of liability, recognized in statement of financial position, for defined benefit pension and other postretirement plans, classified as current.

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Amount of liability, recognized in statement of financial position, for defined benefit pension and other postretirement plans, classified as noncurrent.

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