Summary of Significant Accounting Policies - Additional Information (Details)	1 Months Ended			3 Months Ended	12 Months Ended
	Oct. 31, 2018 USD ($)	May 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2020 USD ($) Station	Sep. 30, 2020 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2020 USD ($) ReportingUnit	Sep. 30, 2020 USD ($)	Sep. 30, 2020 USD ($) operating-segment	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2020 USD ($)	Oct. 25, 2018
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Number of reportable segments							2		2
Revisions in estimated cash flows						$ 192,400,000				$ 1,000,000.0
Difference between replacement cost of gas storage and inventory basis amount						12,500,000				17,000,000.0
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
Goodwill accumulated impairment loss					$ 0	0	0	0	0
Impairment charge of long-lived assets						148,600,000				$ 0	$ 0
Number of months of the original maturity of debt instruments purchased					3 months
Restricted cash											17,000,000.0
ASU 2017-07
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Reclassification of net periodic benefit costs (income) to other income (expense)											$ 14,400,000
ASU 2016-13 | Scenario Forecast
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Credit loss allowances for available-for-sale securities recognized at adoption of ASU												$ 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
Gas Marketing
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Goodwill					$ 0	$ 0	$ 0	$ 0	$ 0
Natural Gas Storage Facility
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Percentage of voting interest acquired	20.00%	100.00%	80.00%
Consideration	$ 17,000,000.0	$ 12,200,000	$ 24,800,000
Payments to acquire business			16,000,000.0
Non-interest-bearing note			10,000,000.0
Non-interest-bearing note, value			$ 8,800,000
Percentage of voting interest													20.00%
Spire Missouri
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Average annual depreciation and amortization expense percentage					2.90%	2.90%	2.90%	2.90%	2.90%	3.00%	3.00%
Revisions in estimated cash flows						$ (25,900,000)				$ (2,800,000)
Goodwill					$ 210,200,000	210,200,000	$ 210,200,000	$ 210,200,000	$ 210,200,000	210,200,000
Spire Missouri | ASU 2017-07
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Reclassification of net periodic benefit costs (income) to other income (expense)											$ 17,200,000
Spire Missouri | ASU 2016-13 | Scenario Forecast
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Credit loss allowances for available-for-sale securities recognized at adoption of ASU												0
Spire Missouri | Affiliated Entity | Spire Marketing | Regulated Operation
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Purchases of natural gas from related party						56,900,000				95,300,000	71,500,000
Sales of natural gas to Spire Marketing Inc.						100,000				$ 1,700,000	$ 300,000
Spire Alabama
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Composite depreciation rate								3.10%		3.10%	3.10%
Revisions in estimated cash flows						221,100,000				$ 0
Finance receivable, net					$ 9,400,000	$ 9,400,000	$ 9,400,000	$ 9,400,000	$ 9,400,000	11,700,000
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
Spire Alabama | ASU 2017-07
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Reclassification of net periodic benefit costs (income) to other income (expense)											$ (2,000,000.0)
Spire Alabama | ASU 2016-13 | Scenario Forecast
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Credit loss allowances for available-for-sale securities recognized at adoption of ASU												$ 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 90 days past due					$ 300,000	$ 300,000	$ 300,000	$ 300,000	$ 300,000	$ 400,000
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						6,300,000
Sales of natural gas to Spire Marketing Inc.						$ 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