INCOME TAXES - Effective Income Tax Rate Variation from Stated Tax Rate (Details)	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Income Tax Contingency [Line Items]
Federal income tax statutory rate	24.50%	35.00%	35.00%
State and local income taxes, net of federal income tax benefits	3.40%	2.80%	2.80%
Certain expenses capitalized on books and deducted on tax return	(2.30%)	(2.30%)	(3.40%)
Taxes related to prior years	(0.40%)	(0.90%)	(0.20%)
Tax law changes	(35.90%)	0.00%	0.00%
Amortization of excess deferred taxes	(1.80%)	0.00%	0.00%
Other items – net	(1.60%)	(2.20%)	(1.70%)
Effective income tax rate	(14.10%)	32.40%	32.50%
Spire Missouri
Income Tax Contingency [Line Items]
Federal income tax statutory rate	24.50%	35.00%	35.00%
State and local income taxes, net of federal income tax benefits	3.40%	2.80%	2.80%
Certain expenses capitalized on books and deducted on tax return	(4.60%)	(3.50%)	(4.80%)
Taxes related to prior years	(0.70%)	(1.40%)	(0.20%)
Tax law changes	(50.30%)	0.00%	0.00%
Amortization of excess deferred taxes	(3.60%)	0.00%	0.00%
Other items – net	(2.50%)	(3.30%)	(2.80%)
Effective income tax rate	(33.80%)	29.60%	30.00%
Spire Alabama
Income Tax Contingency [Line Items]
Federal income tax statutory rate	24.50%	35.00%	35.00%
State and local income taxes, net of federal income tax benefits	3.80%	2.80%	2.80%
Tax law changes	70.00%	0.00%	0.00%
Other items – net	0.10%	0.30%	0.10%
Effective income tax rate	98.40%	38.10%	37.90%