SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Asset Retirement Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Asset retirement obligations [Roll forward]
Asset retirement obligations, beginning of year	$ 159.2	$ 99.2
Liabilities incurred during the period	4.1	2.3
Liabilities settled during the period	(9.5)	(2.0)
Accretion	13.2	4.5
Revisions in estimated cash flows	27.5	55.2
Asset retirement obligations, end of year	206.4	159.2
Laclede Gas
Asset retirement obligations [Roll forward]
Asset retirement obligations, beginning of year	72.4	71.2
Liabilities incurred during the period	1.2	0.6
Liabilities settled during the period	(1.9)	(1.9)
Accretion	3.5	3.4
Revisions in estimated cash flows	0.0	(0.9)
Asset retirement obligations, end of year	75.2	72.4
Alagasco
Asset retirement obligations [Roll forward]
Asset retirement obligations, beginning of year	86.6	27.7
Liabilities incurred during the period	2.9	1.7
Liabilities settled during the period	(6.8)	(0.1)
Accretion	9.7	1.1
Revisions in estimated cash flows	27.7	56.2
Asset retirement obligations, end of year	120.1	86.6
EnergySouth
Asset retirement obligations [Roll forward]
Addition of EnergySouth asset retirement obligations	11.9	0.0
EnergySouth | Laclede Gas
Asset retirement obligations [Roll forward]
Addition of EnergySouth asset retirement obligations	0.0	0.0
EnergySouth | Alagasco
Asset retirement obligations [Roll forward]
Addition of EnergySouth asset retirement obligations	$ 0.0	$ 0.0