Summary of Significant Accounting Policies - Additional Information (Details)	9 Months Ended
	Jun. 30, 2021 USD ($) reporting_unit	Jun. 30, 2021 USD ($) operating-segment
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
Change in accounting principle, accounting standards update, adopted	true	true
Change in accounting principle, accounting standards update, adoption date	Dec. 31, 2020	Dec. 31, 2020
Change in accounting principle, accounting standards update, material effect	true	true
Accounting Standards Update 2016-13 | Spire Missouri
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
Change in accounting principle, accounting standards update, adopted	true	true
Change in accounting principle, accounting standards update, adoption date	Dec. 31, 2020	Dec. 31, 2020
Change in accounting principle, accounting standards update, material effect	true	true
Accounting Standards Update 2016-13 | Spire Alabama
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
Change in accounting principle, accounting standards update, adopted	true	true
Change in accounting principle, accounting standards update, adoption date	Dec. 31, 2020	Dec. 31, 2020
Change in accounting principle, accounting standards update, material effect	true	true