Regulatory Matters - Additional Information (Details) - USD ($) $ in Millions			3 Months Ended		12 Months Ended
	Dec. 11, 2020	Apr. 25, 2018	Dec. 31, 2020	Dec. 31, 2019	Sep. 30, 2020
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
Regulatory assets, current			$ 70.5	$ 67.9	$ 69.5
Spire Missouri
Public Utilities, General Disclosures [Line Items]
Regulatory assets, current			32.1	29.4	32.1
MoPSC
Public Utilities, General Disclosures [Line Items]
Approved rate increase (decrease) amount	$ 64.2				2,780.0
Public utilities requested rate increase decrease amount	$ 111.5
Common equity ratio	54.25%
Requested return rate on equity	9.95%
Infrastructure System Replacement Surcharge | MoPSC
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)
Public utilities requested rate increase decrease amount	$ 47.3
Infrastructure System Replacement Surcharge | MoPSC | Spire Missouri
Public Utilities, General Disclosures [Line Items]
Provision for regulatory liability				2.1
Provision for interest				0.5
After-tax impact on loss contingency provision reducing net income				$ 2.0
Accounting Authority Order | MoPSC
Public Utilities, General Disclosures [Line Items]
Regulatory assets, current			$ 3.9		$ 3.8
Pension Cost | Infrastructure System Replacement Surcharge | MoPSC
Public Utilities, General Disclosures [Line Items]
Regulatory assets recovery		$ (28.8)