Derivative Instruments and Hedging Activities - Additional Information (Details) MMBTU in Millions, $ in Millions	1 Months Ended	3 Months Ended				12 Months Ended
	Aug. 31, 2018 USD ($)	Sep. 30, 2020 USD ($) MMBTU	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2020 USD ($) MMBTU	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)
Derivative [Line Items]
Notional amount of non-exchange traded natural gas commodity contracts (in MMBtu) | MMBTU		278.3				278.3
Notional amount of non-exchange traded natural gas commodity contracts to be settled during fiscal 2021 (in MMBtu) | MMBTU		233.1				233.1
Notional amount of non-exchange traded natural gas commodity contracts to be settled during fiscal 2022 (in MMBtu) | MMBTU		35.7				35.7
Notional amount of non-exchange traded natural gas commodity to be settled during fiscal 2023 (in MMBtu) | MMBTU		5.8				5.8
Notional amount of non-exchange traded natural gas commodity to be settled during fiscal 2024 (in MMBtu) | MMBTU		1.3				1.3
Notional amount of non-exchange traded natural gas commodity to be settled during fiscal 2025 (in MMBtu) | MMBTU		1.6				1.6
Notional amount of non-exchange traded natural gas commodity to be settled during fiscal 2026 (in MMBtu) | MMBTU		0.8				0.8
Loss on termination and settlement of interest rate swap		$ 0.3
Cash margin receivables not offset with derivatives		7.6				$ 7.6	$ 8.1
Spire Missouri
Derivative [Line Items]
Cash margin receivables not offset with derivatives							$ 7.2	$ 7.8
Interest Rate Swap with Fixed Interest Rate of 2.7675%
Derivative [Line Items]
Derivative, term of contract	3 years
Derivative fixed interest rate	2.7675%
Notional amount	$ 100.0
Loss on termination and settlement of interest rate swap				$ 2.5
Interest Rate Swap with Fixed Interest Rate of 3.250%
Derivative [Line Items]
Derivative, term of contract					3 years
Derivative fixed interest rate					3.25%
Notional amount					$ 100.0
Cumulative mark-to-market loss in accumulated other comprehensive loss		$ (53.7)				(53.7)
Interest Rate Swap with Fixed Interest Rate of 0.921% to 1.3105%
Derivative [Line Items]
Notional amount				$ 150.0
Mark-to-market loss						$ (0.5)
Interest Rate Swap with Fixed Interest Rate of 0.761%
Derivative [Line Items]
Derivative, term of contract			10 years
Derivative fixed interest rate			0.761%
Notional amount			$ 40.0
Minimum [Member] | Interest Rate Swap with Fixed Interest Rate of 0.921% to 1.3105%
Derivative [Line Items]
Derivative fixed interest rate				0.921%
Maximum | Interest Rate Swap with Fixed Interest Rate of 0.921% to 1.3105%
Derivative [Line Items]
Derivative fixed interest rate				1.3105%