Income Taxes - Effective Income Tax Rate Variation from Stated Tax Rate (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
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%	24.50%	35.00%
State and local income taxes, net of federal income tax benefits		3.60%	3.40%	2.80%
Certain expenses capitalized on books and deducted on tax return		(3.80%)	(2.30%)	(2.30%)
Taxes related to prior years		0.20%	(0.40%)	(0.90%)
Tax law changes		0.00%	(35.90%)	0.00%
Amortization of excess deferred taxes		(3.80%)	(1.80%)	0.00%
Other items – net		(1.40%)	(1.60%)	(2.20%)
Effective income tax rate		15.80%	(14.10%)	32.40%
Spire Missouri
Income Tax Contingency [Line Items]
Federal income tax statutory rate		21.00%	24.50%	35.00%
State and local income taxes, net of federal income tax benefits		2.60%	3.40%	2.80%
Certain expenses capitalized on books and deducted on tax return		(6.50%)	(4.60%)	(3.50%)
Taxes related to prior years		0.10%	(0.70%)	(1.40%)
Tax law changes		0.00%	(50.30%)	0.00%
Amortization of excess deferred taxes		(6.60%)	(3.60%)	0.00%
Other items – net		(0.30%)	(2.50%)	(3.30%)
Effective income tax rate		10.30%	(33.80%)	29.60%
Spire Alabama
Income Tax Contingency [Line Items]
Federal income tax statutory rate		21.00%	24.50%	35.00%
State and local income taxes, net of federal income tax benefits		4.10%	3.80%	2.80%
Certain expenses capitalized on books and deducted on tax return		0.00%	0.00%	0.00%
Taxes related to prior years		0.00%	0.00%	0.00%
Tax law changes		0.00%	70.00%	0.00%
Amortization of excess deferred taxes		0.00%	0.00%	0.00%
Other items – net		0.20%	0.10%	0.30%
Effective income tax rate		25.30%	98.40%	38.10%