Summary of Significant Accounting Policies - Additional Information (Details)	3 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2020 USD ($) reporting_unit	Dec. 31, 2020 USD ($) operating-segment	Dec. 31, 2019 USD ($)
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Number of reportable segments		2	2
Accounting Standards Update 2016-13
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Credit loss allowances for available-for-sale securities recognized at adoption of ASU	$ 0	$ 0	$ 0
Change in accounting principle, accounting standards update, adopted	true	true	true
Change in accounting principle, accounting standards update, adoption date	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2020
Change in accounting principle, accounting standards update, material effect	true	true	true
Spire Alabama | Accounting Standards Update 2016-13
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Credit loss allowances for available-for-sale securities recognized at adoption of ASU	$ 0	$ 0	$ 0
Change in accounting principle, accounting standards update, adopted	true	true	true
Change in accounting principle, accounting standards update, adoption date	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2020
Change in accounting principle, accounting standards update, material effect	true	true	true
Spire Alabama | Affiliated Entity | Spire Marketing | Regulated Operation
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Purchases of natural gas from related party	$ 4,800,000			$ 3,300,000
Spire Missouri | Accounting Standards Update 2016-13
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Credit loss allowances for available-for-sale securities recognized at adoption of ASU	$ 0	$ 0	$ 0
Change in accounting principle, accounting standards update, adopted	true	true	true
Change in accounting principle, accounting standards update, adoption date	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2020
Change in accounting principle, accounting standards update, material effect	true	true	true