STOCKHOLDERS' EQUITY - Narrative (Details) $ / shares in Units, Equity_Security in Thousands			1 Months Ended		12 Months Ended
	Jun. 11, 2014 USD ($) Equity_Security	Aug. 13, 2013 USD ($)	Apr. 30, 2017 shares	Jun. 30, 2014 USD ($) shares $ / shares	Sep. 30, 2015 USD ($) $ / shares shares	Sep. 30, 2014 USD ($) $ / shares shares	Sep. 30, 2013 USD ($) shares	Nov. 24, 2015 USD ($) shares	Nov. 20, 2015 USD ($) shares	Jun. 20, 2014 shares
Accumulated other comprehensive income (loss), net of income taxes [Line Items]
Number of equity units issued (in shares)	2,875			2,875,000
Proceeds from convertible debt	$ 143,800,000
Stated value per Corporate Unit (in dollars per share) | $ / shares				$ 50
Common stock, par value (in dollars per share) | $ / shares				$ 1.00	$ 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				2.00%
Shares reserved for issuance under the stock purchase contracts (in shares) | shares					4,200,000
SEC effective shelf registration Form S-3 sale and issuance authorization under the dividend reinvestment and stock purchase program (in shares) | shares					168,698					168,698
SEC effective shelf registration Form S-3 remaining and available for issuance under the dividend reinvestment and stock purchase program (in shares) | shares					129,413
Preferred stock shares authorized (in shares) | shares					5,000,000	5,000,000
Issuance of first mortgage bonds		$ 450,000,000
Issuance of common stock to Laclede Group						$ 456,800,000	$ 427,200,000
Laclede Gas
Accumulated other comprehensive income (loss), net of income taxes [Line Items]
Common stock, par value (in dollars per share) | $ / shares					$ 1	$ 1
Preferred stock shares authorized (in shares) | shares					1,480,000	1,480,000
Issuance of first mortgage bonds		$ 450,000,000
Issuance of common stock to Laclede Group						$ 1,100,000	$ 477,200,000
Dividend payment restriction formula component - fixed amount					$ 8,000,000
Retained earnings available to pay dividends in period					891,700,000	$ 936,200,000
MoPSC authorized debt issuance through June 30, 2015 - maximum available					$ 518,000,000
Laclede Gas | Affiliated Entity
Accumulated other comprehensive income (loss), net of income taxes [Line Items]
Issuance of common stock to Laclede Group (in shares) | shares					0	28	11,745
Issuance of common stock to Laclede Group						$ 1,100,000	$ 477,200,000
Subsequent Event
Accumulated other comprehensive income (loss), net of income taxes [Line Items]
SEC effective shelf registration Form S-3 remaining and available for issuance under the dividend reinvestment and stock purchase program (in shares) | shares								123,889	123,889
Subsequent Event | Laclede Gas
Accumulated other comprehensive income (loss), net of income taxes [Line Items]
MoPSC authorized debt issuance - remaining available								$ 369,700,000	$ 369,700,000
Scenario, Forecast | Minimum
Accumulated other comprehensive income (loss), net of income taxes [Line Items]
Issuance of common stock to Laclede Group (in shares) | shares			2,500,000
Scenario, Forecast | Maximum
Accumulated other comprehensive income (loss), net of income taxes [Line Items]
Issuance of common stock to Laclede Group (in shares) | shares			3,100,000
Junior Subordinated Debt
Accumulated other comprehensive income (loss), net of income taxes [Line Items]
Proceeds from convertible debt				$ 143,800,000