Pension Plans and Other Postretirement Benefits - Additional Information (Details)	3 Months Ended				6 Months Ended	9 Months Ended	12 Months Ended
	Jun. 30, 2021 USD ($) plan	Mar. 31, 2021 USD ($) plan	Jun. 30, 2020 USD ($) plan	Mar. 31, 2020 USD ($) plan	Mar. 31, 2020 USD ($)	Jun. 30, 2021 USD ($)	Sep. 30, 2020	Dec. 31, 2020	Sep. 30, 2019
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Medical insurance available until age is reached after early retirement						65 years
Other Postretirement Benefits Plan
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contributions						$ 7,200,000
Other Postretirement Benefits Plan | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Number of Plans | plan	1	1
Settlement benefits paid	$ 7,600,000					0
Settlement loss	$ 2,100,000	$ 3,800,000
Discount rates				3.20%	3.20%		2.95%	3.25%	3.20%
Settlement losses amortized over remaining actuarial life		11 years 8 months 12 days					11 years 4 months 24 days
Anticipated contributions to pension plans for qualified trust						2,700,000
Other Postretirement Benefits Plan | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Number of Plans | plan	2		2	2
Settlement benefits paid	$ 36,800,000		$ 6,000,000.0	$ 59,100,000	$ 0
Settlement loss	$ 9,100,000		$ 2,300,000	$ 21,000,000.0
Discount rates			3.00%						3.20%
Anticipated contributions to pension plans for qualified trust						4,700,000
Other Postretirement Benefits Plan | Spire Missouri | Qualified Plan
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contributions						36,900,000
Other Postretirement Benefits Plan | Spire Missouri | Nonqualified Plan
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contributions						0
Other Postretirement Benefits Plan | Spire Missouri First Plan
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rates							2.85%		3.15%
Other Postretirement Benefits Plan | Spire Missouri Second Plan
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rates							2.75%		3.10%
Other Postretirement Benefits Plan | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contributions						0
Anticipated contributions to pension plans for qualified trust						0
Other Postretirement Benefits Plan | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contributions						0
Anticipated contributions to pension plans for qualified trust						$ 0