Pension Plans and Other Postretirement Benefits - Additional Information (Details)	6 Months Ended
	Mar. 31, 2025 USD ($)
Spire Missouri [Member] | Pension Plan [Member] | Qualified Plan [Member]
Defined benefit plan, plan assets, contributions by employer	$ 26,700,000
Anticipated contributions to pension plans qualified trusts	8,800,000
Spire Missouri [Member] | Pension Plan [Member] | Nonqualified Plan [Member]
Defined benefit plan, plan assets, contributions by employer	0
Spire Missouri [Member] | Other Postretirement Benefits Plan [Member]
Defined benefit plan, plan assets, contributions by employer	0
Anticipated contributions to pension plans qualified trusts	0
Spire Alabama Inc [Member] | Pension Plan [Member]
Defined benefit plan, plan assets, contributions by employer	18,200,000
Anticipated contributions to pension plans qualified trusts	5,500,000
Spire Alabama Inc [Member] | Other Postretirement Benefits Plan [Member]
Defined benefit plan, plan assets, contributions by employer	0
Anticipated contributions to pension plans qualified trusts	$ 0