STOCKHOLDERS' EQUITY - Narrative (Details) $ / shares in Units, Equity_Security in Thousands			1 Months Ended		12 Months Ended
	May 17, 2016 USD ($) shares	Jun. 11, 2014 USD ($) Equity_Security	Apr. 30, 2017 shares	Jun. 30, 2014 USD ($) shares $ / shares	Sep. 30, 2016 USD ($) $ / shares shares	Sep. 30, 2015 USD ($) $ / shares shares	Sep. 30, 2014 USD ($) shares	Nov. 11, 2016 shares	Feb. 10, 2016 USD ($)	Jun. 30, 2015 USD ($)	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%
Issuance of stock (in shares)	2,185,000
Shares reserved for issuance under the stock purchase contracts (in 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)					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)					106,535
Issuance of common stock | $	$ 133,200,000				$ 133,200,000		$ 456,800,000
Preferred stock shares authorized (in shares)					5,000,000	5,000,000
Preferred stock shares issued (in shares)					0	0
Preferred stock shares outstanding (in shares)					0	0
Dividend payment restriction formula component - fixed amount | $					$ 8,000,000.0
Retained earnings available to pay dividends in period | $					$ 916,800,000	$ 891,700,000
Laclede Gas
Accumulated other comprehensive income (loss), net of income taxes [Line Items]
Common stock, par value (in dollars per share) | $ / shares					$ 1	$ 1
Issuance of common stock | $							$ 1,100,000
Preferred stock shares authorized (in shares)					1,480,000	1,480,000
Regulatory preferred stock and debt securities issuance, amount authorized | $					$ 300,000,000				$ 300,000,000.0	$ 518,000,000.0
Laclede Gas | Affiliated Entity
Accumulated other comprehensive income (loss), net of income taxes [Line Items]
Issuance of stock (in shares)					0	0	28
Issuance of common stock | $							$ 1,100,000
Alagasco
Accumulated other comprehensive income (loss), net of income taxes [Line Items]
Common stock, par value (in dollars per share) | $ / shares					$ 0.01	$ 0.01
Preferred stock shares authorized (in shares)					120,000	120,000
Preferred stock shares issued (in shares)					0	0
Preferred stock shares outstanding (in shares)					0	0
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)								101,439
Scenario, Forecast | Minimum
Accumulated other comprehensive income (loss), net of income taxes [Line Items]
Issuance of stock (in shares)			2,500,000
Scenario, Forecast | Maximum
Accumulated other comprehensive income (loss), net of income taxes [Line Items]
Issuance of stock (in 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