NOTES PAYABLE AND CREDIT AGREEMENTS - Narrative (Details)	12 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016	Dec. 14, 2016 bank
Line of Credit Facility [Line Items]
Long-term Line of Credit	$ 477,300,000
Speculative interest rate increase percent	1.00%
Decrease in cash flow with an impact of 100 basis point interest rate increase	$ 4,900,000
Spire Missouri
Line of Credit Facility [Line Items]
Long-term Line of Credit	203,000,000
Long-term debt	$ 975,000,000
Speculative interest rate increase percent	1.00%
Decrease in cash flow with an impact of 100 basis point interest rate increase	$ 2,800,000
Spire Alabama
Line of Credit Facility [Line Items]
Long-term Line of Credit	169,900,000
Long-term debt	$ 250,000,000
Speculative interest rate increase percent	1.00%
Decrease in cash flow with an impact of 100 basis point interest rate increase	$ 1,100,000
Spire Energy South Inc
Line of Credit Facility [Line Items]
Long-term Line of Credit	12,900,000
Spire STL Pipeline LLC
Line of Credit Facility [Line Items]
Long-term Line of Credit	26,600,000
Line of Credit
Line of Credit Facility [Line Items]
Line of credit facility borrowing capacity	$ 975,000,000.0
Minimum required ratio of earnings before interest, taxes, depreciation, and amortization (EBITDA) times interest expense as required by the line of credit covenant	70.00%
Debt to total capitalization ratio	0.56
Line of Credit | Spire Missouri
Line of Credit Facility [Line Items]
Line of credit facility borrowing capacity	$ 475,000,000.0
Debt to total capitalization ratio	0.50
Line of Credit | Spire Alabama
Line of Credit Facility [Line Items]
Line of credit facility borrowing capacity	$ 200,000,000.0
Debt to total capitalization ratio	0.33
Line of Credit | Liabilities | Lender Concentration Risk
Line of Credit Facility [Line Items]
Line Of Credit Facility, Number Of Banks In Agreement | bank			11
Percent of debt by a singe bank		12.30%
Subject to Callable at Par | Spire Missouri
Line of Credit Facility [Line Items]
Long-term debt	$ 520,000,000
Spire | Line of Credit
Line of Credit Facility [Line Items]
Line of credit facility borrowing capacity	300,000,000.0
Other Affiliates
Line of Credit Facility [Line Items]
Long-term Line of Credit	27,600,000
Other subsidiaries
Line of Credit Facility [Line Items]
Long-term Line of Credit	$ 440,000,000