Pension Plans and Other Postretirement Benefits - Additional Information (Details) - USD ($)	3 Months Ended	9 Months Ended
	Jun. 30, 2020	Jun. 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]
Settlement benefits paid	$ 6,000,000.0	$ 59,100,000
Settlement loss	$ 2,300,000	21,000,000.0
Employer contributions		$ 9,100,000
Other Postretirement Benefits Plan | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rates	3.00%	3.00%	3.20%
Anticipated contributions to pension plans for qualified trust		$ 7,800,000
Other Postretirement Benefits Plan | Spire Missouri | Qualified Plan
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contributions		20,900,000
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 Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Anticipated contributions to pension plans for qualified trust		3,600,000
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
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