PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Fair Value Measurements of Plan Assets (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2012
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	$ 9.3		$ 0
Stock/Bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	9.3		0
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	506.6	[1]	345.4	[1]	274.1
Pension Plans | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	10.2		18.2
Pension Plans | Stock/Bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	138.2		115.8
Pension Plans | US bond mutual funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	73.6		17.7
Pension Plans | US government
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	64.5		55.7
Pension Plans | US corporate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	164.0		110.9
Pension Plans | US municipal
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	8.2		6.8
Pension Plans | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	35.5		21.6
Pension Plans | Alternative
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	13.4
Pension Plans | Derivative instruments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	(1.0)	[2]	(1.3)	[3]
Pension Plans | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	136.4		35.9
Pension Plans | Quoted Prices in Active Markets (Level 1) | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	8.6		18.2
Pension Plans | Quoted Prices in Active Markets (Level 1) | Stock/Bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	54.2
Pension Plans | Quoted Prices in Active Markets (Level 1) | US bond mutual funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	73.6		17.7
Pension Plans | Significant Observable Inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	360.9		309.5
Pension Plans | Significant Observable Inputs (Level 2) | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	1.6
Pension Plans | Significant Observable Inputs (Level 2) | Stock/Bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	74.7		115.8
Pension Plans | Significant Observable Inputs (Level 2) | US bond mutual funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets			0
Pension Plans | Significant Observable Inputs (Level 2) | US government
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	64.5		55.7
Pension Plans | Significant Observable Inputs (Level 2) | US corporate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	164.0		110.9
Pension Plans | Significant Observable Inputs (Level 2) | US municipal
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	8.2		6.8
Pension Plans | Significant Observable Inputs (Level 2) | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	35.5		21.6
Pension Plans | Significant Observable Inputs (Level 2) | Alternative
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	13.4
Pension Plans | Significant Observable Inputs (Level 2) | Derivative instruments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	(1.0)	[2]	(1.3)	[3]
Pension Plans | Significant Unobservable Inputs (Level 3)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	9.3
Pension Plans | Significant Unobservable Inputs (Level 3) | Stock/Bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	9.3
Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	222.5	[1]	111.6	[1]	67.4
Postretirement Plans | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	2.3		1.4
Postretirement Plans | Stock/Bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	213.0		110.2
Postretirement Plans | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	7.2
Postretirement Plans | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	222.5		111.6
Postretirement Plans | Quoted Prices in Active Markets (Level 1) | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	2.3		1.4
Postretirement Plans | Quoted Prices in Active Markets (Level 1) | Stock/Bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	213.0		110.2
Postretirement Plans | Quoted Prices in Active Markets (Level 1) | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	7.2
Laclede Gas | Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	387.4		345.4		274.1
Derivative Assets, fair value measurements	2.9		4.2
Cash margin payable	3.9		5.5
Laclede Gas | Pension Plans | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	8.3		18.2
Laclede Gas | Pension Plans | Stock/Bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	48.5		115.8
Laclede Gas | Pension Plans | US bond mutual funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	73.6		17.7
Laclede Gas | Pension Plans | US government
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	60.5		55.7
Laclede Gas | Pension Plans | US corporate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	154.5		110.9
Laclede Gas | Pension Plans | US municipal
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	8.2		6.8
Laclede Gas | Pension Plans | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	34.8		21.6
Laclede Gas | Pension Plans | Derivative instruments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	(1.0)	[2]	(1.3)	[3]
Laclede Gas | Pension Plans | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	81.9		35.9
Laclede Gas | Pension Plans | Quoted Prices in Active Markets (Level 1) | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	8.3		18.2
Laclede Gas | Pension Plans | Quoted Prices in Active Markets (Level 1) | US bond mutual funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	73.6		17.7
Laclede Gas | Pension Plans | Significant Observable Inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	296.2		309.5
Laclede Gas | Pension Plans | Significant Observable Inputs (Level 2) | Stock/Bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	39.2		115.8
Laclede Gas | Pension Plans | Significant Observable Inputs (Level 2) | US bond mutual funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets			0
Laclede Gas | Pension Plans | Significant Observable Inputs (Level 2) | US government
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	60.5		55.7
Laclede Gas | Pension Plans | Significant Observable Inputs (Level 2) | US corporate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	154.5		110.9
Laclede Gas | Pension Plans | Significant Observable Inputs (Level 2) | US municipal
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	8.2		6.8
Laclede Gas | Pension Plans | Significant Observable Inputs (Level 2) | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	34.8		21.6
Laclede Gas | Pension Plans | Significant Observable Inputs (Level 2) | Derivative instruments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	(1.0)	[2]	(1.3)	[3]
Laclede Gas | Pension Plans | Significant Unobservable Inputs (Level 3)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	9.3
Laclede Gas | Pension Plans | Significant Unobservable Inputs (Level 3) | Stock/Bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	9.3
Laclede Gas | Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	137.2		111.6		67.4
Laclede Gas | Postretirement Plans | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	2.3		1.4
Laclede Gas | Postretirement Plans | Stock/Bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	134.9		110.2
Laclede Gas | Postretirement Plans | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	137.2		111.6
Laclede Gas | Postretirement Plans | Quoted Prices in Active Markets (Level 1) | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	2.3		1.4
Laclede Gas | Postretirement Plans | Quoted Prices in Active Markets (Level 1) | Stock/Bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	$ 134.9		$ 110.2

[1] Includes Alagasco.

[2] Derivative assets of $2.9 net of cash margin payable of $3.9.

[3] Derivative assets of $4.2 net of cash margin payable of $5.5.