PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Net Periodic Cost (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period	$ 4.3	$ 2.5	$ 8.6	$ 4.9
Interest cost on projected benefit obligation	7.4	6.0	14.9	12.0
Expected return on plan assets	(9.3)	(6.7)	(18.7)	(13.3)
Amortization of prior service cost	0.1	0.1	0.2	0.2
Amortization of actuarial loss	2.0	1.7	3.9	3.5
Loss on lump-sum settlements	0	1.3	0	1.3
Sub-total	4.5	4.9	8.9	8.6
Regulatory adjustment	4.8	1.6	9.7	4.5
Net pension cost	9.3	6.5	18.6	13.1
Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period	3.2	2.8	6.4	5.6
Interest cost on projected benefit obligation	2.8	2.2	5.6	4.4
Expected return on plan assets	(3.3)	(1.7)	(6.6)	(3.4)
Amortization of prior service cost	0.2	0	0.4	0
Amortization of actuarial loss	1.2	1.5	2.5	3.0
Sub-total	4.1	4.8	8.3	9.6
Regulatory adjustment	(2.7)	(2.4)	(5.4)	(4.8)
Net pension cost	1.4	2.4	2.9	4.8
Laclede Gas | Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period	2.9	2.5	5.7	4.9
Interest cost on projected benefit obligation	5.9	6.0	11.8	12.0
Expected return on plan assets	(7.3)	(6.7)	(14.6)	(13.3)
Amortization of prior service cost	0.1	0.1	0.2	0.2
Amortization of actuarial loss	2.0	1.7	3.9	3.5
Loss on lump-sum settlements	0	1.3	0	1.3
Sub-total	3.6	4.9	7.0	8.6
Regulatory adjustment	3.4	1.6	6.8	4.5
Net pension cost	7.0	6.5	13.8	13.1
Laclede Gas | Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period	3.1	2.8	6.2	5.6
Interest cost on projected benefit obligation	2.2	2.2	4.3	4.4
Expected return on plan assets	(2.1)	(1.7)	(4.1)	(3.4)
Amortization of prior service cost	0.2	0	0.4	0
Amortization of actuarial loss	1.2	1.5	2.5	3.0
Sub-total	4.6	4.8	9.3	9.6
Regulatory adjustment	(2.3)	(2.4)	(4.6)	(4.8)
Net pension cost	2.3	2.4	4.7	4.8
Alagasco | Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period	1.4	1.5	2.9	3.6
Interest cost on projected benefit obligation	1.5	1.2	3.1	2.9
Expected return on plan assets	(2.0)	(1.5)	(4.1)	(3.7)
Amortization of prior service cost	0	0.1	0	0.1
Amortization of actuarial loss	0	0.7	0	2.0
Loss on lump-sum settlements	0	10.1	0	10.9
Sub-total	0.9	12.1	1.9	15.8
Regulatory adjustment	1.4	(9.8)	2.9	(10.6)
Net pension cost	2.3	2.3	4.8	5.2
Alagasco | Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period	0.1	0.1	0.2	0.4
Interest cost on projected benefit obligation	0.6	0.5	1.3	1.2
Expected return on plan assets	(1.2)	(1.1)	(2.5)	(2.1)
Amortization of transition obligation	0	0.1	0	0.3
Amortization of actuarial loss	0	(0.4)	0	(0.5)
Sub-total	(0.5)	(0.8)	(1.0)	(0.7)
Regulatory adjustment	(0.4)	0	(0.8)	0
Net pension cost	$ (0.9)	$ (0.8)	$ (1.8)	$ (0.7)