Summary of Significant Accounting Policies - Additional Information (Details)	3 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($) Station	Jun. 30, 2020 USD ($) Station	Sep. 30, 2021 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2021 USD ($) ReportingUnit	Sep. 30, 2021 USD ($)	Sep. 30, 2021 USD ($) operating-segment	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Number of reportable segments					2		2
Revisions in estimated cash flows				$ (31,500,000)				$ 192,400,000
Difference between replacement cost of gas storage and inventory basis amount				14,000,000.0				12,500,000
Goodwill	$ 1,171,600,000		$ 1,171,600,000	1,171,600,000	$ 1,171,600,000	$ 1,171,600,000	$ 1,171,600,000	1,171,600,000
Goodwill accumulated impairment loss	0		$ 0	0	0	0	0
Impairment charge of long-lived assets				0				$ 148,600,000	$ 0
Number of months of the original maturity of debt instruments purchased			3 months
Restricted cash	$ 7,000,000.0		$ 7,000,000.0	$ 7,000,000.0	$ 7,000,000.0	$ 7,000,000.0	$ 7,000,000.0
Accounting Standards Update 2016-02
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Change in accounting principle, accounting standards update, adopted								true
Change in accounting principle, accounting standards update, adoption date								Oct. 01, 2019
Accounting Standards Update 2017-12
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Change in accounting principle, accounting standards update, adopted								true
Change in accounting principle, accounting standards update, adoption date								Oct. 01, 2019
Change in accounting principle, accounting standards update, immaterial effect								true
Accounting Standards Update 2016-13
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Change in accounting principle, accounting standards update, adopted	true		true	true	true	true	true
Change in accounting principle, accounting standards update, adoption date	Dec. 31, 2020		Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2020
Change in accounting principle, accounting standards update, immaterial effect	true		true	true	true	true	true
Credit loss allowances for available-for-sale securities recognized at adoption of ASU	$ 0		$ 0	$ 0	$ 0	$ 0	$ 0
Gas Utility
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Goodwill	210,200,000		210,200,000	210,200,000	210,200,000	210,200,000	210,200,000
Gas Marketing
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Goodwill	0		0	0	0	$ 0	0
Spire Alabama
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Composite depreciation rate						3.00%		3.00%	3.00%
Revisions in estimated cash flows				(23,200,000)				$ 221,100,000
Finance receivable, net	$ 7,800,000		$ 7,800,000	$ 7,800,000	$ 7,800,000	$ 7,800,000	$ 7,800,000	$ 9,400,000
Threshold period past due for collection			90 days
Threshold period past due for write-off of financing receivable	12 months		12 months	12 months	12 months	12 months	12 months
Spire Alabama | Accounting Standards Update 2016-02
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Change in accounting principle, accounting standards update, adopted								true
Change in accounting principle, accounting standards update, adoption date								Oct. 01, 2019
Spire Alabama | Accounting Standards Update 2017-12
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Change in accounting principle, accounting standards update, adopted								true
Change in accounting principle, accounting standards update, adoption date								Oct. 01, 2019
Change in accounting principle, accounting standards update, immaterial effect								true
Spire Alabama | Accounting Standards Update 2016-13
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Change in accounting principle, accounting standards update, adopted	true		true	true	true	true	true
Change in accounting principle, accounting standards update, adoption date	Dec. 31, 2020		Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2020
Change in accounting principle, accounting standards update, immaterial effect	true		true	true	true	true	true
Credit loss allowances for available-for-sale securities recognized at adoption of ASU	$ 0		$ 0	$ 0	$ 0	$ 0	$ 0
Spire Alabama | Financing Receivables, Equal to Greater than 90 Days Past Due
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Finance receivable	300,000		300,000	300,000	300,000	$ 300,000	300,000	$ 300,000
Spire Missouri
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Composite depreciation rate						3.00%		3.00%	3.00%
Revisions in estimated cash flows				(7,900,000)				$ (25,900,000)
Goodwill	$ 210,200,000		$ 210,200,000	$ 210,200,000	$ 210,200,000	$ 210,200,000	$ 210,200,000	$ 210,200,000
Spire Missouri | Accounting Standards Update 2016-02
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Change in accounting principle, accounting standards update, adopted								true
Change in accounting principle, accounting standards update, adoption date								Oct. 01, 2019
Spire Missouri | Accounting Standards Update 2017-12
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Change in accounting principle, accounting standards update, adopted								true
Change in accounting principle, accounting standards update, adoption date								Oct. 01, 2019
Change in accounting principle, accounting standards update, immaterial effect								true
Spire Missouri | Accounting Standards Update 2016-13
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Change in accounting principle, accounting standards update, adopted	true		true	true	true	true	true
Change in accounting principle, accounting standards update, adoption date	Dec. 31, 2020		Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2020
Change in accounting principle, accounting standards update, immaterial effect	true		true	true	true	true	true
Credit loss allowances for available-for-sale securities recognized at adoption of ASU	$ 0		$ 0	$ 0	$ 0	$ 0	$ 0
Spire Storage West LLC ("Spire Storage")
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Number of fueling stations sold | Station	1
Gain recorded on sale fueling stations	$ 1,300,000
Spire Storage West LLC ("Spire Storage") | Non-Utility Property
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Impairment charge of long-lived assets		$ 140,800,000
Number of commercial stations impaired | Station		2
Spire Storage West LLC ("Spire Storage") | Two Compressed Natural Gas Fueling Stations (Non-utility Property)
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Impairment charge of long-lived assets		$ 7,800,000