Income Taxes - Effective Income Tax Rate Variation from Stated Tax Rate (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Income Tax Contingency [Line Items]
Federal income tax statutory rate	35.00%	21.00%
Spire
Income Tax Contingency [Line Items]
Federal income tax statutory rate		21.00%	21.00%	24.50%
State and local income taxes, net of federal income tax benefits		9.00%	3.60%	3.40%
Certain expenses capitalized on books and deducted on tax return		(6.60%)	(3.80%)	(2.30%)
Taxes related to prior years		(1.80%)	0.20%	(0.40%)
Tax law changes		0.00%	0.00%	(35.90%)
Amortization of excess deferred taxes		(8.30%)	(3.80%)	(1.80%)
Other items – net		(1.00%)	(1.40%)	(1.60%)
Effective income tax rate		12.30%	15.80%	(14.10%)
Spire Missouri
Income Tax Contingency [Line Items]
Federal income tax statutory rate		21.00%	21.00%	24.50%
State and local income taxes, net of federal income tax benefits		2.60%	2.60%	3.40%
Certain expenses capitalized on books and deducted on tax return		(4.60%)	(6.50%)	(4.60%)
Taxes related to prior years		(1.40%)	0.10%	(0.70%)
Tax law changes		0.00%	0.00%	(50.30%)
Amortization of excess deferred taxes		(5.70%)	(6.60%)	(3.60%)
Other items – net		(0.20%)	(0.30%)	(2.50%)
Effective income tax rate		11.70%	10.30%	(33.80%)
Spire Alabama
Income Tax Contingency [Line Items]
Federal income tax statutory rate		21.00%	21.00%	24.50%
State and local income taxes, net of federal income tax benefits		4.10%	4.10%	3.80%
Certain expenses capitalized on books and deducted on tax return		0.00%	0.00%	0.00%
Taxes related to prior years		0.10%	0.00%	0.00%
Tax law changes		0.00%	0.00%	70.00%
Amortization of excess deferred taxes		0.00%	0.00%	0.00%
Other items – net		(0.10%)	0.20%	0.10%
Effective income tax rate		25.10%	25.30%	98.40%