Income Taxes - Impact of Tax Cuts and Jobs Act (Details) $ in Millions	12 Months Ended
	Sep. 30, 2018 USD ($)
Spire
Deferred Income Tax Assets And Liabilities [Line Items]
Adjustment to deferred tax liabilities	$ (318.3)
Adjustment to deferred income tax expense	(75.0)
Adjustment to regulatory assets	(75.9)
Adjustment to regulatory liabilities	167.4
Spire Missouri
Deferred Income Tax Assets And Liabilities [Line Items]
Adjustment to deferred tax liabilities	(285.3)
Adjustment to deferred income tax expense	(57.0)
Adjustment to regulatory assets	(78.1)
Adjustment to regulatory liabilities	150.2
Spire Alabama
Deferred Income Tax Assets And Liabilities [Line Items]
Adjustment to deferred tax liabilities	(61.0)
Adjustment to deferred income tax expense	58.8
Adjustment to regulatory assets	$ 2.2