LONG-TERM DEBT - Narrative (Details) - USD ($)					9 Months Ended	12 Months Ended
	Dec. 01, 2015	Jan. 15, 2015	Aug. 19, 2014	Dec. 06, 2013	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Nov. 24, 2015	Nov. 20, 2015	Sep. 15, 2015	Mar. 31, 2015	Feb. 03, 2015	Nov. 03, 2014	Jun. 20, 2014	Jun. 11, 2014	Jan. 06, 2014
Debt Instrument [Line Items]
Long-term debt issued amount			$ 625,000,000.0
Stated interest rate on debt issued																	2.00%
Fixed-rate long-term debt amount						$ 1,603,800,000
Long-term debt amount bearing variable interest rate						250,000,000
Long-term debt						$ 1,710,000,000
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)						129,413
Percent of common stock equity					44.90%	47.00%	44.90%
Percent of long-term debt					55.10%	53.00%	55.10%
Repayment of long-term debt						$ 34,800,000	$ 80,000,000		$ 25,000,000
Debt instrument, principal outstanding, excluding current portion					$ 1,851,000,000	1,771,500,000	$ 1,851,000,000
Laclede Gas
Debt Instrument [Line Items]
Long-term debt						$ 810,000,000
Percent of common stock equity					55.50%	56.20%	55.50%
Percent of long-term debt					44.50%	43.80%	44.50%
Repayment of long-term debt						$ 0	$ 80,000,000		$ 25,000,000
MoPSC authorized debt issuance through June 30, 2015 - maximum available						518,000,000
Debt instrument, principal outstanding, excluding current portion					$ 807,900,000	808,100,000	$ 807,900,000
Alagasco
Debt Instrument [Line Items]
Long-term debt						$ 250,000,000
Percent of common stock equity					77.30%	83.70%	77.30%
Percent of long-term debt					22.70%	16.30%	22.70%
Long-term debt reclassified to current						$ 80,000,000
Repayment of long-term debt					$ 0	34,800,000		$ 0
Long-term debt amount authorized														$ 80,000,000.0	$ 35,000,000.0
Debt instrument, principal outstanding, excluding current portion					$ 249,800,000	170,000,000	$ 249,800,000
Debt instrument redeemed		$ 34,800,000
Unsecured debt													$ 115,000,000.0
Alagasco | Unsecured notes
Debt Instrument [Line Items]
Long-term debt issued amount						115,000,000.0
Long-term debt reclassified to current						80,000,000
Floating Rate Senior Notes, due August 15, 2017 | Senior Notes
Debt Instrument [Line Items]
Long-term debt issued amount			250,000,000.0
2.55% Senior Notes, due August 15, 2019 | Senior Notes
Debt Instrument [Line Items]
Long-term debt issued amount			$ 125,000,000.0
Stated interest rate on debt issued			2.55%
4.70% Senior Notes, due August 15, 2044 | Senior Notes
Debt Instrument [Line Items]
Long-term debt issued amount			$ 250,000,000.0
Stated interest rate on debt issued			4.70%
No Call Options
Debt Instrument [Line Items]
Long-term debt						25,000,000
No Call Options | Laclede Gas
Debt Instrument [Line Items]
Long-term debt						25,000,000
Make Whole Call Options
Debt Instrument [Line Items]
Long-term debt						710,000,000
Make Whole Call Options | Laclede Gas
Debt Instrument [Line Items]
Long-term debt						$ 435,000,000
2.0% Series A Remarketable Subordinated Notes, due April 1, 2022 | Junior Subordinated Debt
Debt Instrument [Line Items]
Stated interest rate on debt issued						2.00%
Long-term debt						$ 143,800,000
6.35% Series, due June 1, 2029 | Laclede Gas | Mortgages
Debt Instrument [Line Items]
Stated interest rate on debt issued																		6.35%
Repayment of long-term debt				$ 80,000,000
5.7% Notes, due January 15, 2035 | Alagasco
Debt Instrument [Line Items]
Stated interest rate on debt issued					5.70%	5.70%	5.70%								5.70%
Debt instrument, principal outstanding, excluding current portion															$ 34,800,000
5.368% Notes, due December 1, 2015 | Alagasco
Debt Instrument [Line Items]
Long-term debt issued amount														$ 80,000,000
Stated interest rate on debt issued					5.368%	5.368%	5.368%
3.21% Notes, due September 15, 2025 | Alagasco
Debt Instrument [Line Items]
Stated interest rate on debt issued					3.21%	3.21%	3.21%
Unsecured debt												$ 35,000,000.0
3.21% Notes, due September 15, 2025 | Alagasco | Unsecured notes
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
Three-Month LIBOR | Floating Rate Senior Notes, due August 15, 2017 | Senior Notes
Debt Instrument [Line Items]
Description of variable rate basis			three-month LIBOR
Basis spread on variable rate			0.75%
One to six months prior to maturity | Subject to Callable at Par
Debt Instrument [Line Items]
Long-term debt						$ 725,000,000
Redemption period prior to maturity, minimum						1 month
Redemption period prior to maturity, maximum						6 months
One year prior to maturity | Subject to Callable at Par
Debt Instrument [Line Items]
Long-term debt						$ 250,000,000
Redemption period prior to maturity						1 year
Three to six months prior to maturity | Subject to Callable at Par | Laclede Gas
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
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)										123,889	123,889
Subsequent Event | Laclede Gas
Debt Instrument [Line Items]
MoPSC authorized debt issuance - remaining available										$ 369,700,000	$ 369,700,000
Scenario, Forecast | Notes 4.31%, Due December 1, 2045 | Alagasco
Debt Instrument [Line Items]
Unsecured debt	$ 80,000,000.0
Scenario, Forecast | Notes 4.31%, Due December 1, 2045 | Alagasco | Unsecured notes
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