Pension Plans and Other Postretirement Benefits - Amounts Recognized in Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent liabilities	$ (235.9)	$ (309.0)
Spire Missouri
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent liabilities	(159.5)	(217.2)
Spire Alabama
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent liabilities	(66.7)	(74.9)
Pension Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Current liabilities	(0.8)	(0.6)
Noncurrent liabilities	(189.9)	(258.9)
Total	(190.7)	(259.5)
Pension Plans | Spire Missouri
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Current liabilities	(0.8)	(0.6)
Noncurrent liabilities	(114.2)	(168.4)
Total	(115.0)	(169.0)
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	(66.6)	(74.9)
Total	(66.6)	(74.9)
Other Postretirement Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent assets	170.2	129.0
Current liabilities	(0.5)	(0.5)
Noncurrent liabilities	(47.0)	(49.8)
Total	122.7	78.7
Other Postretirement Plans | Spire Missouri
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent assets	117.5	90.3
Current liabilities	(0.5)	(0.5)
Noncurrent liabilities	(47.0)	(49.0)
Total	70.0	40.8
Other Postretirement Plans | Spire Alabama
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent assets	51.4	38.7
Current liabilities	0.0	0.0
Noncurrent liabilities	0.0	0.0
Total	$ 51.4	$ 38.7