INCOME TAXES - Net Provisions for Income Taxes (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
Federal
Current		$ (3.3)	$ 0.3		$ (4.2)
Deferred		58.8	30.6		19.9
Investment tax credits		(0.2)	(0.2)		(0.2)
State and local
Current		0.0	0.6		(0.3)
Deferred		6.9	1.0		2.4
Total income tax expense		62.2	32.3		17.6
Laclede Gas
Federal
Current		(2.1)	(0.1)		(6.6)
Deferred		40.9	34.3		20.1
Investment tax credits		(0.2)	(0.2)		(0.2)
State and local
Current		(0.1)	0.0		(1.0)
Deferred		4.7	1.5		2.3
Total income tax expense		43.2	$ 35.5		$ 14.6
Alagasco
Federal
Current	$ 14.1	0.0		$ 17.5
Deferred	3.5	25.9		13.3
State and local
Current	1.8	0.1		2.2
Deferred	0.5	3.3		1.7
Total income tax expense	$ 19.9	$ 29.3		$ 34.7