Pension Plans and Other Postretirement Benefits - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Sep. 30, 2017
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
Pension Plans
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Settlement benefits paid	$ 34,500,000	$ 39,500,000
Settlement loss	7,500,000	$ 9,400,000
Pension Plans | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Settlement benefits paid	19,600,000
Settlement loss	$ 5,200,000
Discount rates	4.20%			3.70%
Employer contributions			$ 18,300,000
Anticipated contributions to pension plans for qualified trust			8,400,000
Pension Plans | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Settlement benefits paid	$ 14,900,000
Settlement loss	$ 2,300,000
Discount rates	4.20%			3.65%
Other Pension Plan | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rates	4.15%			3.75%
Postretirement Plans
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contributions			0
Postretirement Plans | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contributions			0
Postretirement Plans | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Anticipated contributions to pension plans for qualified trust			$ 0