Pension Plans and Other Postretirement Benefits - Amounts Recognized in Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent liabilities	$ (309.0)	$ (264.8)
Spire Missouri
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent liabilities	(217.2)	(192.4)
Spire Alabama
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent liabilities	(74.9)	(59.2)
Pension Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Current liabilities	(0.6)	(0.5)
Noncurrent liabilities	(258.9)	(229.1)
Total	(259.5)	(229.6)
Pension Plans | Spire Missouri
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Current liabilities	(0.6)	(0.5)
Noncurrent liabilities	(168.4)	(158.7)
Total	(169.0)	(159.2)
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	(74.9)	(59.2)
Total	(74.9)	(59.2)
Other Postretirement Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent assets	129.0	118.3
Current liabilities	(0.5)	(0.5)
Noncurrent liabilities	(49.8)	(35.3)
Total	78.7	82.5
Other Postretirement Plans | Spire Missouri
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent assets	90.3	75.3
Current liabilities	(0.5)	(0.5)
Noncurrent liabilities	(49.0)	(33.8)
Total	40.8	41.0
Other Postretirement Plans | Spire Alabama
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent assets	38.7	43.0
Current liabilities	0.0	0.0
Noncurrent liabilities	0.0	0.0
Total	$ 38.7	$ 43.0