PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Fair Value of Plan Assets (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2012
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	$ 0
Fair value of plan assets, end of year	9.3		0
Lump-sum payments recognized as settlements	22.1		79.5
Pension Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	345.4	[1]	274.1
Actual return on plan assets	52.1	[1]	3.4
Employer contributions	23.6	[1]	28.0
Gross benefits paid	(36.9)	[1],[2]	(87.0)	[2]
Fair value of plan assets, end of year	506.6	[1]	345.4	[1]	274.1
Funded status of plans, end of year	(185.9)	[1]	(158.4)
Lump-sum payments recognized as settlements	22.1		79.5		60.1
Postretirement Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	111.6	[1]	67.4
Actual return on plan assets	11.6	[1]	5.6
Employer contributions	19.1	[1]	16.6
Gross benefits paid	(7.3)	[1]	(8.4)
Fair value of plan assets, end of year	222.5	[1]	111.6	[1]
Funded status of plans, end of year	(36.0)	[1]	(68.5)
Laclede Gas | Pension Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	345.4		274.1
Actual return on plan assets	55.0		3.4
Employer contributions	23.6		28.0
Gross benefits paid	(36.6)	[2]	(87.0)	[2]
Fair value of plan assets, end of year	387.4		345.4
Funded status of plans, end of year	(156.2)		(158.4)
Laclede Gas | Postretirement Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	111.6		67.4
Actual return on plan assets	13.3		5.6
Employer contributions	19.1		16.6
Gross benefits paid	(6.8)		(8.4)
Fair value of plan assets, end of year	137.2		111.6
Funded status of plans, end of year	(60.7)		(68.5)
Missouri Gas Energy (MGE) | Pension Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
MGE acquisition	0	[1]	126.9
Missouri Gas Energy (MGE) | Postretirement Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
MGE acquisition	0	[1]	30.4
Missouri Gas Energy (MGE) | Laclede Gas | Pension Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
MGE acquisition	0		126.9
Missouri Gas Energy (MGE) | Laclede Gas | Postretirement Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
MGE acquisition	0		30.4
Alagasco | Pension Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
MGE acquisition	122.4	[1]	0
Alagasco | Postretirement Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
MGE acquisition	87.5	[1]	0
Alagasco | Laclede Gas | Pension Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
MGE acquisition	0		0
Alagasco | Laclede Gas | Postretirement Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
MGE acquisition	$ 0		$ 0

[1] Includes Alagasco.

[2] Includes $22.1 and $79.5 lump-sum payments recognized as settlements in fiscal years 2014 and 2013, respectively.