INCOME TAXES - Impact of Tax Cuts and Jobs Act (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Deferred Income Tax Assets And Liabilities [Line Items]
Adjustment to deferred tax assets	$ 0.0
Adjustment to deferred tax liabilities	(299.5)
Adjustment to deferred income tax expense	(69.4)
Adjustment to regulatory assets	(59.4)
Adjustment to regulatory liabilities	170.7
Spire Missouri
Deferred Income Tax Assets And Liabilities [Line Items]
Adjustment to deferred tax assets	0.0
Adjustment to deferred tax liabilities	(268.2)
Adjustment to deferred income tax expense	(54.6)
Adjustment to regulatory assets	(61.0)
Adjustment to regulatory liabilities	152.6
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