STOCKHOLDER'S EQUITY - Narrative (Details) (USD $)	0 Months Ended	1 Months Ended	12 Months Ended				0 Months Ended	1 Months Ended	12 Months Ended				0 Months Ended	1 Months Ended					12 Months Ended				12 Months Ended				12 Months Ended
	Jun. 11, 2014	Jun. 30, 2014	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012	Jun. 11, 2014	Aug. 13, 2013 Laclede Gas	Aug. 30, 2012 Laclede Gas	Sep. 30, 2014 Laclede Gas	Sep. 30, 2013 Laclede Gas	Sep. 30, 2012 Laclede Gas	Nov. 23, 2012 Laclede Gas	Jun. 11, 2014 Junior Subordinated Debt	Jun. 30, 2014 Junior Subordinated Debt	Apr. 30, 2017 Scenario, Forecast Minimum	Apr. 30, 2017 Scenario, Forecast Maximum	Jun. 30, 2014 Common Stock Issued	May 31, 2013 Common Stock Issued	Sep. 30, 2014 Common Stock Issued	Sep. 30, 2013 Common Stock Issued	Sep. 30, 2012 Common Stock Issued	Sep. 30, 2011 Common Stock Issued	Sep. 30, 2014 Common Stock Issued Laclede Gas	Sep. 30, 2013 Common Stock Issued Laclede Gas	Sep. 30, 2012 Common Stock Issued Laclede Gas	Sep. 30, 2011 Common Stock Issued Laclede Gas	Sep. 30, 2014 Paid-in Capital	Sep. 30, 2013 Paid-in Capital	Sep. 30, 2012 Paid-in Capital	Sep. 30, 2014 Paid-in Capital Laclede Gas	Sep. 30, 2013 Paid-in Capital Laclede Gas	Sep. 30, 2012 Paid-in Capital Laclede Gas	Sep. 30, 2014 Affiliated Entity Laclede Gas	Sep. 30, 2013 Affiliated Entity Laclede Gas	Sep. 30, 2013 Missouri Gas Energy (MGE) Laclede Gas	Nov. 21, 2014 Subsequent Event
Accumulated other comprehensive income (loss), net of income taxes [Line Items]
Common stock, issued (in shares)			43,200,000	32,700,000					24,577	24,549									43,178,405	32,696,836	22,539,431	22,430,734	24,577	24,549	12,804	11,717
Number of Equity Units issued (in shares)	2,875,000	2,875,000
Proceeds from convertible debt													$ 143,800,000	$ 143,800,000
Stated value per Corporate Unit (in dollars per share)		$ 50
Common stock, par value (in dollars per share)		$ 1.00	$ 1	$ 1					$ 1	$ 1
Percentage of beneficial ownership interest in Subordinated Notes						5.00%
Stated principal amount of Subordinated Notes						1,000
Interest rate on Subordinated borrowing ( in percent)		2.00%
Issuance of common stock to Laclede Group															2,500,000	3,100,000			10,350,000	10,005,000			28	11,745	1,087								28	11,745
Shares reserved for issuance under the stock purchase contracts (in shares)		4,200,000
Increase in common stock and paid-in capital			445,600,000	435,800,000					5,900,000	480,800,000
Common stock issued to fund acquisitions		456,800,000
Common stock issued to fund acquisitions (in shares)																	10,350,000
Shares issued - dividend reinvestment plan and stock purchase plan (in shares)				44,074													33,667		33,667	44,074	46,107
Shares issued - dividend reinvestment plan and stock purchase plan			1,500,000	1,800,000	1,900,000																100,000						1,500,000	1,800,000	1,800,000
Stock issued under the Equity Incentive Plan (in shares)																			97,902	108,331	62,590
Shares issued - Equity plan and stock-based compensation costs			7,000,000	434,000,000
Stock issued during period related to stock-based compensation (in shares)																		10,005,000
Stock issued costs related to purchase contract liability	19,700,000		19,700,000																								19,700,000
SEC effective shelf registration Form S-3 sale and issuance authorization under the dividend reinvestment and stock purchase program (in shares)			168,698
SEC effective shelf registration Form S-3 remaining and available for issuance under the dividend reinvestment and stock purchase program (in shares)			160,579																																	152,810
Issuance of first mortgage bonds							450,000,000
Preferred stock shares authorized (in shares)			5,000,000	1,480,000					5,000,000	1,480,000
Issuance of common stock to Laclede Group			456,800,000	427,200,000					1,100,000	477,200,000	42,700,000								10,400,000	10,000,000							446,400,000	417,200,000		1,100,000	477,200,000	42,700,000	1,100,000	476,500,000
Acquisition purchase price			(23,900,000)	975,000,000	0				(23,900,000)	975,000,000	0																								475,000,000
Dividend payment restriction formula component - fixed amount									8,000,000
Retained earnings available to pay dividends in period									936,200,000	833,000,000
Regulatory Preferred Stock and Debt Securities Issuance Amount Authorized												518,000,000
Extension period for debt securities and preferred stock issuance authority								2 years
MoPSC authorized debt issuance - remaining available									$ 369,700,000