INCOME TAXES - Impact of Tax Cuts and Jobs Act (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Deferred Income Tax Assets And Liabilities [Line Items]
Adjustment to deferred tax assets	$ 0.0
Adjustment to deferred tax liabilities	(296.6)
Adjustment to deferred income tax expense	(59.9)	$ 0.0
Adjustment to regulatory assets	(59.4)
Adjustment to regulatory liabilities	177.3
Spire Missouri
Deferred Income Tax Assets And Liabilities [Line Items]
Adjustment to deferred tax assets	0.0
Adjustment to deferred tax liabilities	(264.1)
Adjustment to deferred income tax expense	(43.9)
Adjustment to regulatory assets	(61.0)
Adjustment to regulatory liabilities	159.2
Spire Alabama
Deferred Income Tax Assets And Liabilities [Line Items]
Adjustment to deferred tax assets	(60.8)
Adjustment to deferred tax liabilities	0.0
Adjustment to deferred income tax expense	59.2
Adjustment to regulatory assets	1.6
Adjustment to regulatory liabilities	$ 0.0