Pension Plans and Other Postretirement Benefits - Additional Information (Details)	3 Months Ended
	Dec. 31, 2019 USD ($)
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	$ 1,600,000
Other Postretirement Benefits Plan | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Anticipated contributions to pension plans for qualified trust	24,900,000
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	11,500,000
Other Postretirement Benefits Plan | Qualified Plan | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contributions	4,100,000
Other Postretirement Benefits Plan | Nonqualified Plan | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contributions	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
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