Regulatory Matters - Schedule of Regulatory Assets and Liabilities, Reflected in Balance Sheets (Details) - USD ($) $ in Millions	Mar. 31, 2019	Sep. 30, 2018	Mar. 31, 2018
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	$ 75.3	$ 72.8	$ 97.7
Regulatory assets, non current	659.9	669.8	673.6
Total Regulatory Assets	735.2	742.6	771.3
Regulatory liabilities, current	35.3	35.7	22.2
Regulatory liabilities, non current	431.3	354.6	353.1
Total Regulatory Liabilities	466.6	390.3	375.3
Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	30.0	30.7	51.7
Regulatory assets, non current	433.6	441.1	446.6
Total Regulatory Assets	463.6	471.8	498.3
Regulatory liabilities, current	24.2	16.7	2.7
Regulatory liabilities, non current	345.1	274.9	272.5
Total Regulatory Liabilities	369.3	291.6	275.2
Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	30.4	26.2	32.1
Regulatory assets, non current	198.9	201.5	198.9
Total Regulatory Assets	229.3	227.7	231.0
Regulatory liabilities, current	6.1	7.6	10.8
Regulatory liabilities, non current	30.5	31.3	35.0
Total Regulatory Liabilities	36.6	38.9	45.8
Pension and Postretirement Benefit Costs
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	30.2	30.2	43.0
Regulatory assets, non current	345.8	364.9	358.2
Pension and Postretirement Benefit Costs | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	21.9	21.9	34.9
Regulatory assets, non current	277.9	292.5	279.0
Pension and Postretirement Benefit Costs | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	7.3	7.3	7.1
Regulatory assets, non current	61.1	64.8	71.2
Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	19.3	8.2	32.6
Unamortized Purchased Gas Adjustments | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	0.6	1.0	13.5
Unamortized Purchased Gas Adjustments | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	18.0	6.4	19.0
Other
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	25.8	34.4	22.1
Regulatory assets, non current	45.3	42.4	46.1
Other | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	7.5	7.8	3.3
Regulatory assets, non current	24.7	21.4	24.9
Other | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	5.1	12.5	6.0
Regulatory assets, non current	3.2	3.3	2.7
Future Income Taxes Due from Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	102.2	96.3	113.0
Future Income Taxes Due from Customers | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	99.0	94.4	111.4
Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	134.6	133.4	125.0
Accrued Cost of Removal | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	134.6	133.4	125.0
Energy Efficiency
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	32.0	32.8	31.3
Energy Efficiency | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	32.0	32.8	31.3
Pension and Postretirement Benefit Costs
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	5.8	2.2	2.2
Regulatory liabilities, non current	115.3	27.8	30.8
Pension and Postretirement Benefit Costs | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	3.6	0.0	0.0
Regulatory liabilities, non current	79.4	0.0	0.0
Pension and Postretirement Benefit Costs | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	2.2	2.2	2.2
Regulatory liabilities, non current	26.5	27.8	30.8
Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	3.4	2.9	1.5
Regulatory liabilities, non current	75.7	4.7	50.3
Unamortized Purchased Gas Adjustments | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	2.7	1.9	0.0
Regulatory liabilities, non current	75.7	4.7	50.3
Other
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	26.1	30.6	18.5
Regulatory liabilities, non current	30.9	80.2	30.3
Other | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	17.9	14.8	2.7
Regulatory liabilities, non current	24.9	69.3	20.9
Other | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	3.9	5.4	8.6
Regulatory liabilities, non current	4.0	3.5	4.2
Deferred Taxes Due to Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	155.7	178.3	170.7
Deferred Taxes Due to Customers | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	138.3	161.1	152.6
Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	53.7	63.6	71.0
Accrued Cost of Removal | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	$ 26.8	$ 39.8	$ 48.7