LONG-TERM DEBT - Narrative (Details) (USD $)	12 Months Ended						0 Months Ended	12 Months Ended			0 Months Ended														0 Months Ended				0 Months Ended				0 Months Ended				0 Months Ended				0 Months Ended				0 Months Ended		0 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012	Aug. 19, 2014	Jun. 11, 2014 Junior Subordinated Debt	Sep. 30, 2014 Alagasco	Aug. 13, 2013 Laclede Gas	Sep. 30, 2014 Laclede Gas	Sep. 30, 2013 Laclede Gas	Nov. 23, 2012 Laclede Gas	Aug. 13, 2013 Laclede Gas Mortgages	Mar. 15, 2013 Laclede Gas Mortgages	Aug. 13, 2013 Laclede Gas Mortgages	Aug. 19, 2014 Floating Rate Senior Notes, due August 15, 2017 Senior Notes	Aug. 19, 2014 Senior Notes 2.55%, due August 15, 2019 Senior Notes	Aug. 19, 2014 Senior Notes 4.70%, due August 15, 2044 Senior Notes	Sep. 30, 2014 No Call Options	Sep. 30, 2014 No Call Options Laclede Gas	Sep. 30, 2014 Make Whole Call Options	Sep. 30, 2014 Make Whole Call Options Alagasco	Sep. 30, 2014 Make Whole Call Options Laclede Gas	Sep. 30, 2014 2.0% Series A Remarketable Subordinated Notes, due April 1, 2022 Junior Subordinated Debt	Sep. 30, 2014 3% Series, due March 15, 2023 Laclede Gas	Sep. 30, 2013 3% Series, due March 15, 2023 Laclede Gas	Mar. 15, 2013 3% Series, due March 15, 2023 Laclede Gas Mortgages	Mar. 15, 2013 3% Series, due March 15, 2023 Laclede Gas Mortgages	Sep. 30, 2014 3.40% Series, due March 15, 2028 Laclede Gas	Sep. 30, 2013 3.40% Series, due March 15, 2028 Laclede Gas	Mar. 15, 2013 3.40% Series, due March 15, 2028 Laclede Gas Mortgages	Mar. 15, 2013 3.40% Series, due March 15, 2028 Laclede Gas Mortgages	Sep. 30, 2014 2% Series, due August 15, 2018 Laclede Gas	Sep. 30, 2013 2% Series, due August 15, 2018 Laclede Gas	Aug. 13, 2013 2% Series, due August 15, 2018 Laclede Gas Mortgages	Aug. 13, 2013 2% Series, due August 15, 2018 Laclede Gas Mortgages	Sep. 30, 2014 3.40% Series, due August 15, 2023 Laclede Gas	Sep. 30, 2013 3.40% Series, due August 15, 2023 Laclede Gas	Aug. 13, 2013 3.40% Series, due August 15, 2023 Laclede Gas Mortgages	Aug. 13, 2013 3.40% Series, due August 15, 2023 Laclede Gas Mortgages	Sep. 30, 2014 4.625% Series, due August 15, 2043 Laclede Gas	Sep. 30, 2013 4.625% Series, due August 15, 2043 Laclede Gas	Aug. 13, 2013 4.625% Series, due August 15, 2043 Laclede Gas Mortgages	Aug. 13, 2013 4.625% Series, due August 15, 2043 Laclede Gas Mortgages	Sep. 30, 2014 6.35% Series, due October 15, 2038 Laclede Gas	Sep. 30, 2013 6.35% Series, due October 15, 2038 Laclede Gas	Jan. 06, 2014 6.35% Series, due October 15, 2038 Laclede Gas Mortgages	Jan. 06, 2014 6.35% Series, due October 15, 2038 Laclede Gas Mortgages	Aug. 19, 2014 Three-Month LIBOR Floating Rate Senior Notes, due August 15, 2017 Senior Notes	Sep. 30, 2014 Three to six months prior to maturity Subject to Callable at Par	Sep. 30, 2014 Three to six months prior to maturity Subject to Callable at Par Laclede Gas	Sep. 30, 2014 One year prior to maturity Subject to Callable at Par	Sep. 30, 2014 30 days' notice Subject to Callable at Par	Sep. 30, 2014 30 days' notice Subject to Callable at Par Alagasco	Nov. 03, 2014 Subsequent Event Alagasco
Debt Instrument [Line Items]
Long-term debt issued amount				$ 625,000,000.0									$ 450,000,000.0	$ 250,000,000.0	$ 125,000,000.0	$ 250,000,000.0
Description of variable rate basis																																															three-month LIBOR
Basis spread on variable rate (in percent)																																															0.75%
Stated interest rate on debt issued					2.00%										2.55%	4.70%							3.00%	3.00%		300.00%	3.40%	3.40%		3.40%	2.00%	2.00%		2.00%	3.40%	3.40%		340.00%	4.625%	4.625%		4.625%	6.35%	6.35%		6.35%
Long-term Debt	1,853,600,000																25,000,000		675,000,000			143,800,000																										725,000,000		250,000,000	34,800,000
Fixed-rate long-term debt amount	1,603,600,000					249,800,000		810,000,000										25,000,000		215,000,000	435,000,000																												350,000,000			34,800,000	35,000,000
Fixed-rate long-term debt, interest rate (in percent)																																																				5.70%
Long-term debt amount bearing variable interest rate	250,000,000
Maximum redemption amount per year																																																			800,000	800,000
Issuance of first mortgage bonds							450,000,000				450,000,000	100,000,000													55,000,000				45,000,000				100,000,000				250,000,000				100,000,000
Debt issued, maturity term																									10 years				15 years
Repayment of long-term debt	80,000,000	0	0					80,000,000																																					80,000,000
MoPSC authorized debt issuance through June 30, 2013 - maximum available										$ 518,000,000
Preferred stock shares authorized (in shares)	5,000,000	1,480,000						5,000,000	1,480,000