Pension Plans and Other Postretirement Benefits - Additional Information (Details)	3 Months Ended		6 Months Ended
	Mar. 31, 2021 USD ($) plan	Mar. 31, 2020 USD ($) plan	Mar. 31, 2021 USD ($) plan	Sep. 30, 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			$ 4,500,000
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	$ 12,900,000		$ 0
Settlement loss	$ 3,800,000
Discount rates				2.95%	3.20%
Settlement losses amortized over remaining actuarial life			11 years 4 months 24 days
Anticipated contributions to pension plans for qualified trust			$ 6,600,000
Other Postretirement Benefits Plan | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Number of Plans | plan		2
Settlement benefits paid		$ 59,100,000
Settlement loss		$ 21,000,000.0
Discount rates				2.85%	3.20%
Anticipated contributions to pension plans for qualified trust			13,700,000
Other Postretirement Benefits Plan | Spire Missouri | Qualified Plan
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contributions			28,300,000
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 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