INCOME TAXES - Unrecognized Tax Benefit Reconciliation (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits, beginning of year		$ 7.1	$ 4.6	$ 2.4
Increases related to tax positions taken in current year		3.4	2.9	2.6
Reductions due to lapse of applicable statute of limitations		(0.5)	(0.4)	(0.4)
Unrecognized tax benefits, end of year	$ 4.6	10.0	7.1	4.6
Laclede Gas
Unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits, beginning of year		6.9	4.2	2.0
Increases related to tax positions taken in current year		3.3	2.9	2.5
Reductions due to lapse of applicable statute of limitations		(0.5)	(0.2)	(0.3)
Unrecognized tax benefits, end of year	4.2	9.7	6.9	4.2
Alagasco
Unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits, beginning of year	0.3	0.0	0.0
Reductions due to lapse of applicable statute of limitations	(0.3)	0.0	0.0
Unrecognized tax benefits, end of year	$ 0.0	$ 0.0	$ 0.0	$ 0.0