LONG-TERM DEBT - Narrative (Details) - USD ($)				12 Months Ended
	Dec. 01, 2015	Sep. 15, 2015	Jan. 15, 2015	Sep. 30, 2016	Nov. 11, 2016	Sep. 09, 2016	Feb. 10, 2016	Sep. 30, 2015	Jun. 30, 2015	Feb. 03, 2015	Nov. 03, 2014	Aug. 19, 2014	Jun. 20, 2014	Jun. 11, 2014
Debt Instrument [Line Items]
Long-term debt issued amount												$ 625,000,000.0
Stated interest rate on debt issued														2.00%
Long-term debt amount bearing variable interest rate				$ 250,000,000
Long-term debt				$ 2,085,800,000
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
Percent of common stock equity				49.10%				47.00%
Percent of long-term debt				50.90%				53.00%
Debt instrument, principal outstanding, excluding current portion				$ 1,833,700,000				$ 1,771,500,000
Subsequent Event
Debt Instrument [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
Senior Notes
Debt Instrument [Line Items]
Long-term debt				1,942,000,000
Senior Notes | Unsecured Series 2016 Senior Notes
Debt Instrument [Line Items]
Long-term debt issued amount						$ 165,000,000.0
Senior Notes | Unsecured Series 2016 Senior Notes, Tranche A
Debt Instrument [Line Items]
Long-term debt issued amount						$ 35,000,000
Stated interest rate on debt issued						2.52%
Senior Notes | Unsecured Series 2016 Senior Notes, Tranche B
Debt Instrument [Line Items]
Long-term debt issued amount						$ 130,000,000
Stated interest rate on debt issued						3.13%
Senior Notes | No Call Options
Debt Instrument [Line Items]
Long-term debt				25,000,000
Senior Notes | Make Whole Call Options
Debt Instrument [Line Items]
Long-term debt				937,000,000
Senior Notes | One to six months prior to maturity | Subject to Callable at Par
Debt Instrument [Line Items]
Long-term debt				$ 730,000,000
Redemption period prior to maturity, minimum				1 month
Redemption period prior to maturity, maximum				6 months
Senior Notes | One year prior to maturity | Subject to Callable at Par
Debt Instrument [Line Items]
Long-term debt				$ 250,000,000
Junior Subordinated Debt | 2.0% Series A Remarketable Subordinated Notes, due April 1, 2022
Debt Instrument [Line Items]
Stated interest rate on debt issued				2.00%
Long-term debt				$ 143,800,000
Laclede Gas
Debt Instrument [Line Items]
Long-term debt				$ 810,000,000
Percent of common stock equity				56.90%				56.20%
Percent of long-term debt				43.10%				43.80%
Regulatory preferred stock and debt securities issuance, amount authorized				$ 300,000,000			$ 300,000,000.0		$ 518,000,000.0
Debt instrument, principal outstanding, excluding current portion				808,300,000				$ 808,100,000
Laclede Gas | No Call Options
Debt Instrument [Line Items]
Long-term debt				25,000,000
Laclede Gas | Make Whole Call Options
Debt Instrument [Line Items]
Long-term debt				435,000,000
Laclede Gas | Three to six months prior to maturity | Subject to Callable at Par
Debt Instrument [Line Items]
Long-term debt				$ 350,000,000
Redemption period prior to maturity, minimum				3 months
Redemption period prior to maturity, maximum				6 months
Laclede Gas | Subsequent Event
Debt Instrument [Line Items]
Regulatory preferred stock and debt securities issuance, authorized debt issuance - remaining available					$ 300,000,000
Alagasco
Debt Instrument [Line Items]
Long-term debt				$ 250,000,000
Percent of common stock equity				77.60%				83.70%
Percent of long-term debt				22.40%				16.30%
Long-term debt reclassified to current				$ 250,000,000
Long-term debt amount authorized										$ 80,000,000.0	$ 35,000,000.0
Debt instrument, principal outstanding, excluding current portion				250,000,000				$ 170,000,000
Debt instrument redeemed			$ 34,800,000
Alagasco | Make Whole Call Options
Debt Instrument [Line Items]
Long-term debt				$ 250,000,000
Alagasco | 5.7% Notes, due January 15, 2035
Debt Instrument [Line Items]
Stated interest rate on debt issued											5.70%
Debt instrument, principal outstanding, excluding current portion											$ 34,800,000
Alagasco | 5.368% Notes, due December 1, 2015
Debt Instrument [Line Items]
Long-term debt issued amount										$ 80,000,000
Alagasco | 3.21% Notes, due September 15, 2025
Debt Instrument [Line Items]
Stated interest rate on debt issued				3.21%				3.21%
Alagasco | Notes 4.31%, Due December 1, 2045
Debt Instrument [Line Items]
Stated interest rate on debt issued				4.31%				4.31%
Alagasco | Unsecured notes
Debt Instrument [Line Items]
Long-term debt issued amount				$ 115,000,000.0
Alagasco | Unsecured notes | 3.21% Notes, due September 15, 2025
Debt Instrument [Line Items]
Long-term debt issued amount		$ 35,000,000.0
Stated interest rate on debt issued		3.21%
Debt issued, maturity term		10 years
Alagasco | Unsecured notes | Notes 4.31%, Due December 1, 2045
Debt Instrument [Line Items]
Long-term debt issued amount	$ 80,000,000.0
Stated interest rate on debt issued	4.31%
Debt issued, maturity term	30 years
Other subsidiaries
Debt Instrument [Line Items]
Long-term debt				$ 67,000,000