Regulatory Matters - Schedule of Regulatory Assets and Liabilities, Reflected in Balance Sheets (Details) - USD ($) $ in Millions	Jun. 30, 2019	Sep. 30, 2018	Jun. 30, 2018
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	$ 79.0	$ 72.8	$ 96.7
Regulatory assets, non current	657.1	669.8	695.1
Total Regulatory Assets	736.1	742.6	791.8
Regulatory liabilities, current	31.3	35.7	24.5
Regulatory liabilities, non current	396.3	354.6	364.3
Total Regulatory Liabilities	427.6	390.3	388.8
Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	30.2	30.7	55.1
Regulatory assets, non current	429.2	441.1	473.8
Total Regulatory Assets	459.4	471.8	528.9
Regulatory liabilities, current	23.4	16.7	8.7
Regulatory liabilities, non current	310.5	274.9	281.6
Total Regulatory Liabilities	333.9	291.6	290.3
Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	33.9	26.2	26.9
Regulatory assets, non current	199.9	201.5	193.4
Total Regulatory Assets	233.8	227.7	220.3
Regulatory liabilities, current	3.4	7.6	8.8
Regulatory liabilities, non current	29.8	31.3	33.7
Total Regulatory Liabilities	33.2	38.9	42.5
Pension and Postretirement Benefit Costs
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	30.1	30.2	43.2
Regulatory assets, non current	335.7	364.9	369.4
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	269.8	292.5	297.2
Pension and Postretirement Benefit Costs | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	7.3	7.3	7.3
Regulatory assets, non current	59.3	64.8	64.5
Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	18.9	8.2	14.6
Unamortized Purchased Gas Adjustments | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	0.8	1.0	7.0
Unamortized Purchased Gas Adjustments | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	17.6	6.4	7.6
Other
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	30.0	34.4	38.9
Regulatory assets, non current	45.7	42.4	37.2
Other | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	7.5	7.8	13.2
Regulatory assets, non current	25.5	21.4	16.4
Other | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	9.0	12.5	12.0
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	105.7	96.3	130.5
Future Income Taxes Due from Customers | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	101.3	94.4	128.4
Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	137.4	133.4	126.2
Accrued Cost of Removal | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	137.4	133.4	126.2
Energy Efficiency
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	32.6	32.8	31.8
Energy Efficiency | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	32.6	32.8	31.8
Pension and Postretirement Benefit Costs
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	5.8	2.2	2.2
Regulatory liabilities, non current	119.8	27.8	30.2
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	84.6	0.0	0.0
Pension and Postretirement Benefit Costs | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	2.3	2.2	2.2
Regulatory liabilities, non current	25.8	27.8	30.2
Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	3.3	2.9	1.1
Regulatory liabilities, non current	47.9	4.7	37.0
Unamortized Purchased Gas Adjustments | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	2.5	1.9	0.0
Regulatory liabilities, non current	47.9	4.7	37.0
Other
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	22.2	30.6	21.2
Regulatory liabilities, non current	29.9	80.2	49.4
Other | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	17.3	14.8	8.7
Regulatory liabilities, non current	24.1	69.3	37.8
Other | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	1.1	5.4	6.6
Regulatory liabilities, non current	4.0	3.5	3.5
Deferred Taxes Due to Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	152.9	178.3	182.7
Deferred Taxes Due to Customers | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	135.5	161.1	164.6
Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	45.8	63.6	65.0
Accrued Cost of Removal | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	$ 18.4	$ 39.8	$ 42.2