ACQUISITIONS - Summary of Consideration Paid, Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Millions	Sep. 30, 2016	Sep. 12, 2016	Sep. 30, 2015	Sep. 30, 2014	Aug. 31, 2014	Sep. 30, 2013
Business Acquisition [Line Items]
Goodwill	$ 1,164.9		$ 946.0	$ 937.8		$ 247.1
Alagasco
Business Acquisition [Line Items]
Utility plant					$ 892.7
Cash					12.1
Other current assets					99.4
Deferred tax assets					282.0
Other assets					143.4
Long-term debt					(249.8)
Current portion of long-term debt					(15.0)
Other current liabilities					(173.4)
Deferred tax liabilities					0.0
Other liabilities					(130.4)
Total identifiable net assets					861.0
Goodwill					735.8
Deferred tax elimination (Spire)					(271.3)
Consideration (cash)					$ 1,325.5
EnergySouth
Business Acquisition [Line Items]
Utility plant		$ 199.5
Cash		2.0
Other current assets		17.5
Deferred tax assets		0.0
Other assets		79.8
Long-term debt		(67.0)
Current portion of long-term debt		0.0
Other current liabilities		(42.7)
Deferred tax liabilities		(35.5)
Other liabilities		(52.8)
Total identifiable net assets		100.8
Goodwill		218.9
Deferred tax elimination (Spire)		0.0
Consideration (cash)		$ 319.7