Income Taxes - Effective Income Tax Rate Variation from Stated Tax Rate (Details)	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Spire
Income Tax Contingency [Line Items]
Federal income tax statutory rate	21.00%	21.00%	21.00%
State and local income taxes, net of federal income tax benefits	3.60%	9.00%	3.60%
Certain expenses capitalized on books and deducted on tax return	(1.60%)	(6.60%)	(3.80%)
Taxes related to prior years	(0.50%)	(1.80%)	0.20%
Amortization of excess deferred taxes	(2.50%)	(8.30%)	(3.80%)
Other items ? net	0.10%	(1.00%)	(1.40%)
Effective income tax rate	20.10%	12.30%	15.80%
Spire Missouri
Income Tax Contingency [Line Items]
Federal income tax statutory rate	21.00%	21.00%	21.00%
State and local income taxes, net of federal income tax benefits	2.60%	2.60%	2.60%
Certain expenses capitalized on books and deducted on tax return	(3.30%)	(4.60%)	(6.50%)
Taxes related to prior years	(0.20%)	(1.40%)	0.10%
Amortization of excess deferred taxes	(5.00%)	(5.70%)	(6.60%)
Other items ? net	(0.20%)	(0.20%)	(0.30%)
Effective income tax rate	14.90%	11.70%	10.30%
Spire Alabama
Income Tax Contingency [Line Items]
Federal income tax statutory rate	21.00%	21.00%	21.00%
State and local income taxes, net of federal income tax benefits	4.10%	4.10%	4.10%
Certain expenses capitalized on books and deducted on tax return	0.00%	0.00%	0.00%
Taxes related to prior years	0.00%	0.10%	0.00%
Amortization of excess deferred taxes	0.00%	0.00%	0.00%
Other items ? net	0.20%	(0.10%)	0.20%
Effective income tax rate	25.30%	25.10%	25.30%