Revenue - Schedule of Operating Revenue Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Disaggregation Of Revenue [Line Items]
Total Operating Revenues	$ 803.5	$ 813.4	$ 1,405.5	$ 1,375.2
Spire Missouri
Disaggregation Of Revenue [Line Items]
Total operating revenues	565.7		983.4
Changes in accrued revenue under alternative revenue programs	(9.1)		(13.6)
Total Operating Revenues	556.6	533.2	969.8	925.5
Spire Missouri | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues	410.9		713.0
Spire Missouri | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues	129.2		218.3
Spire Missouri | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues	9.9		18.9
Spire Missouri | Off-system & Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues	11.5		26.4
Spire Missouri | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues	4.2		6.8
Spire Alabama
Disaggregation Of Revenue [Line Items]
Total operating revenues	185.1		319.7
Changes in accrued revenue under alternative revenue programs	(4.7)		(5.8)
Total Operating Revenues	180.4	$ 218.3	313.9	$ 339.1
Spire Alabama | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues	116.9		192.3
Spire Alabama | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues	44.1		75.5
Spire Alabama | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues	18.8		37.0
Spire Alabama | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 5.3		$ 14.9