Revenue - Schedule of Revenue Disaggregated by Source and Customer Type (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Feb. 28, 2021	Dec. 31, 2021	Dec. 31, 2020
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 3.2
Total operating revenues		$ 555.4	$ 512.6
Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues		(12.6)	(27.1)
Gas Utility | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues		503.5	498.2
Gas Marketing | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues		47.9	24.8
Spire
Disaggregation Of Revenue [Line Items]
Total operating revenues		555.4	512.6
Spire | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues		568.0	539.7
Spire | Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues		(12.6)	(27.1)
Spire | Gas Utility | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues		485.4	496.6
Changes in accrued revenue under alternative revenue programs		18.1	1.6
Total operating revenues		503.5	498.2
Spire | Gas Utility | Operating Segments | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues		342.0	349.9
Spire | Gas Utility | Operating Segments | Commercial And Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues		103.8	103.0
Spire | Gas Utility | Operating Segments | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues		31.2	32.2
Spire | Gas Utility | Operating Segments | Off System And Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues		4.8	9.3
Spire | Gas Utility | Operating Segments | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues		3.6	2.2
Spire | Gas Marketing | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues		47.9	24.8
Spire | Other | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues		$ 16.6	$ 16.7