Regulatory Matters - Spire Missouri - Additional Information (Details) $ / shares in Units, $ in Millions						12 Months Ended
	Nov. 16, 2019 USD ($)	Jul. 15, 2019 USD ($)	Mar. 15, 2019 USD ($)	Apr. 25, 2018 USD ($)	Mar. 07, 2018 USD ($) $ / shares	Sep. 30, 2019 USD ($) operating-segment	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Apr. 18, 2018
Public Utilities, General Disclosures [Line Items]
Customer Share						75.00%
Regulatory costs						$ 0.0	$ 30.6	$ 0.0
Minimum
Public Utilities, General Disclosures [Line Items]
Customer Share									70.00%
Maximum
Public Utilities, General Disclosures [Line Items]
Customer Share									85.00%
MoPSC | Infrastructure System Replacement Surcharge
Public Utilities, General Disclosures [Line Items]
Approved rate increase (decrease), costs from buildings sold in 2014 excluded from rate base				$ (1.8)
Rate case expenses				(0.9)
Regulatory costs							$ 38.4
After-tax rate reduction to net income					$ 23.6
After-tax rate reduction to earnings per share | $ / shares					$ 0.49
MoPSC | Infrastructure System Replacement Surcharge | Pension Cost
Public Utilities, General Disclosures [Line Items]
Regulatory assets recovery				$ (28.8)
Regulatory costs			$ 30.6
Spire Missouri
Public Utilities, General Disclosures [Line Items]
Number of PGA Discretionary Changes | operating-segment						3
Months between PGA Discretionary Changes						2 months
Customer Share						25.00%
Spire Missouri | MoPSC
Public Utilities, General Disclosures [Line Items]
Cost savings retention percentage						10.00%
Maximum amount of cost savings allowable						$ 3.0
Spire Missouri | MoPSC | Infrastructure System Replacement Surcharge
Public Utilities, General Disclosures [Line Items]
Approved rate additional annualized revenue		$ 8.8
Spire Missouri | MoPSC | Infrastructure System Replacement Surcharge | Subsequent Event
Public Utilities, General Disclosures [Line Items]
Approved rate authorized future annualized revenue	$ 29.2