PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Benefit Obligation Rollforward (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2012
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Lump-sum payments recognized as settlements	$ 22.1		$ 79.5
Pension Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	503.8	[1]	412.2
Service cost	10.2	[1]	9.2		9.2
Interest cost	24.5	[1]	17.0		19.4
Actuarial (gain) loss	39.4	[1]	(24.0)
Settlement loss	1.2	[1]	25.0
Gross benefits paid	(36.9)	[1],[2]	(87.0)	[2]
Benefit obligation, end of year	692.5	[1]	503.8	[1]	412.2
Accumulated benefit obligation, end of year	613.7	[1]	444.1
Lump-sum payments recognized as settlements	22.1		79.5		60.1
Postretirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	180.1	[1]	127.2
Service cost	11.3	[1]	10.2		8.1
Interest cost	8.9	[1]	5.2		5.5
Actuarial (gain) loss	1.2	[1]	17.5
Plan amendments	2.5	[1]	0
Gross benefits paid	(7.3)	[1]	(8.4)
Benefit obligation, end of year	258.5	[1]	180.1	[1]	127.2
Laclede Gas | Pension Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	503.8		412.2
Service cost	9.7		9.2		9.2
Interest cost	24.0		17.0		19.4
Actuarial (gain) loss	41.5		(24.0)
Settlement loss	1.2		25.0
Gross benefits paid	(36.6)	[2]	(87.0)	[2]
Benefit obligation, end of year	543.6		503.8		412.2
Accumulated benefit obligation, end of year	484.1		444.1
Laclede Gas | Postretirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	180.1		127.2
Service cost	11.2		10.2		8.1
Interest cost	8.7		5.2		5.5
Actuarial (gain) loss	2.2		17.5
Plan amendments	2.5		0
Gross benefits paid	(6.8)		(8.4)
Benefit obligation, end of year	197.9		180.1		127.2
Missouri Gas Energy (MGE) | Pension Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
MGE acquisition	0	[1]	151.4
Missouri Gas Energy (MGE) | Postretirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
MGE acquisition	0	[1]	28.4
Missouri Gas Energy (MGE) | Laclede Gas | Pension Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
MGE acquisition	0		151.4
Missouri Gas Energy (MGE) | Laclede Gas | Postretirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
MGE acquisition	0		28.4
Alagasco | Pension Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
MGE acquisition	150.3	[1]	0
Alagasco | Postretirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
MGE acquisition	61.8	[1]	0
Alagasco | Laclede Gas | Pension Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
MGE acquisition	0		0
Alagasco | Laclede Gas | Postretirement Plans
Defined Benefit Plan, Change in Benefit Obligation [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.