INCOME TAXES - Effective Income Tax Rate Variation from Stated Tax Rate (Details)	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Income Tax Contingency [Line Items]
Federal income tax statutory rate	35.00%	35.00%	35.00%
State and local income taxes, net of federal income tax benefits	2.80%	2.80%	3.00%
Certain expenses capitalized on books and deducted on tax return	(2.30%)	(3.40%)	(3.70%)
Taxes related to prior years	(0.90%)	(0.20%)	(0.60%)
Other items – net	(2.20%)	(1.70%)	(2.50%)
Effective income tax rate	32.40%	32.50%	31.20%
Spire Missouri
Income Tax Contingency [Line Items]
Federal income tax statutory rate	35.00%	35.00%	35.00%
State and local income taxes, net of federal income tax benefits	2.80%	2.80%	2.80%
Certain expenses capitalized on books and deducted on tax return	(3.50%)	(4.80%)	(4.90%)
Taxes related to prior years	(1.40%)	(0.20%)	(0.80%)
Other items – net	(3.30%)	(2.80%)	(3.00%)
Effective income tax rate	29.60%	30.00%	29.10%
Spire Alabama
Income Tax Contingency [Line Items]
Federal income tax statutory rate	35.00%	35.00%	35.00%
State and local income taxes, net of federal income tax benefits	2.80%	2.80%	2.80%
Other items – net	0.30%	0.10%	0.10%
Effective income tax rate	38.10%	37.90%	37.90%