Regulatory Matters - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions						1 Months Ended		3 Months Ended	12 Months Ended
	Feb. 03, 2020	Mar. 15, 2019	Oct. 08, 2018	Apr. 25, 2018	Mar. 07, 2018	Dec. 31, 2019	Apr. 30, 2018	Dec. 31, 2019	Sep. 30, 2018	Sep. 30, 2019
Public Utilities, General Disclosures [Line Items]
Remaining recovery period for certain regulatory assets for which no return on investment during recovery period is provided (in years)								20 years
Remaining recovery period for regulatory assets for which no return on investment during recovery period is provided (in years)								15 years
APSC | Spire Alabama
Public Utilities, General Disclosures [Line Items]
Requested rate increase						$ 5.9
GPSC | Spire Gulf
Public Utilities, General Disclosures [Line Items]
Requested rate increase						$ 1.6
Infrastructure System Replacement Surcharge | Spire Missouri
Public Utilities, General Disclosures [Line Items]
Estimated regulatory liabilities										$ 12.2
Refund ordered amounts collected prior to last rate case							$ 4.2
Infrastructure System Replacement Surcharge | Spire Missouri | Subsequent Event
Public Utilities, General Disclosures [Line Items]
Approved rate authorized future annualized revenue	$ 5.3
Infrastructure System Replacement Surcharge | MoPSC
Public Utilities, General Disclosures [Line Items]
Regulatory costs									$ 38.4
After-tax rate reduction to net income					$ 23.6
After-tax rate reduction to earnings per share					$ 0.49
Approved rate increase (decrease), costs from buildings sold in 2014 excluded from rate base				$ (1.8)
Rate case expenses				(0.9)
Infrastructure System Replacement Surcharge | MoPSC | Spire Missouri
Public Utilities, General Disclosures [Line Items]
Approved rate authorized future annualized revenue								$ 29.2
Approved estimate revenue			$ 8.0
Additional provision for regulatory liability								2.1
Provision for interest								0.5
After-tax impact on loss contingency provision reducing net income								$ (2.0)
Infrastructure System Replacement Surcharge | MoPSC | Spire Missouri | Subsequent Event
Public Utilities, General Disclosures [Line Items]
Approved rate authorized future annualized revenue	$ 16.3
Pension Cost | Infrastructure System Replacement Surcharge | MoPSC
Public Utilities, General Disclosures [Line Items]
Regulatory costs		$ 30.6
Regulatory assets recovery				$ (28.8)