PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Changes in Plan Assets and Benefit Obligations (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2014		Sep. 30, 2013	Sep. 30, 2012
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial (gain) loss	$ 15.7	[1]	$ 17.0	$ 32.9
Amortization of actuarial loss	(7.1)	[1]	(10.7)	(29.1)
Acceleration of loss recognized due to settlement	(1.5)	[1]	(27.0)	0
Amortization of prior service cost	(0.5)	[1]	(0.5)	(0.6)
Sub-total	6.6	[1]	(21.2)	3.2
Regulatory adjustment	(6.1)	[1]	21.1	(3.5)
Total recognized in other comprehensive income	0.5	[1]	(0.1)	(0.3)
Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial (gain) loss	(3.1)	[1]	16.3	10.1
Amortization of actuarial loss	(6.0)	[1]	(5.3)	(4.3)
Amortization of prior service cost	2.5	[1]	0	2.1
Amortization of transition obligation	0	[1]	(0.1)	(0.1)
Sub-total	(6.6)	[1]	10.9	7.8
Regulatory adjustment	6.6	[1]	(10.9)	(7.8)
Total recognized in other comprehensive income	0	[1]	0	0
Laclede Gas | Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial (gain) loss	14.2		17.0	32.9
Amortization of actuarial loss	(7.1)		(10.7)	(29.1)
Acceleration of loss recognized due to settlement	(1.5)		(27.0)	0
Amortization of prior service cost	(0.5)		(0.5)	(0.6)
Sub-total	5.1		(21.2)	3.2
Regulatory adjustment	(4.7)		21.1	(3.5)
Total recognized in other comprehensive income	0.4		(0.1)	(0.3)
Laclede Gas | Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial (gain) loss	(4.2)		16.3	10.1
Amortization of actuarial loss	(6.0)		(5.3)	(4.3)
Amortization of prior service cost	2.5		0	2.1
Amortization of transition obligation	0		(0.1)	(0.1)
Sub-total	(7.7)		10.9	7.8
Regulatory adjustment	7.7		(10.9)	(7.8)
Total recognized in other comprehensive income	$ 0		$ 0	$ 0

[1] Includes Alagasco.