PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Net Periodic Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period	$ 5.3	$ 3.9
Interest cost on projected benefit obligation	6.9	7.1
Expected return on plan assets	(9.9)	(8.9)
Amortization of prior service cost (credit)	0.2	0.1
Amortization of actuarial gain (loss)	3.4	2.0
Special termination benefits	0.0	1.6
Subtotal	5.9	5.8
Regulatory adjustment	4.6	5.0
Net pension cost (income)	10.5	10.8
Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period	2.8	2.8
Interest cost on projected benefit obligation	2.1	2.5
Expected return on plan assets	(3.4)	(3.4)
Amortization of prior service cost (credit)	0.0	0.1
Amortization of actuarial gain (loss)	0.6	0.9
Special termination benefits	0.0	2.6
Subtotal	2.1	5.5
Regulatory adjustment	(0.8)	(4.2)
Net pension cost (income)	1.3	1.3
Laclede Gas | Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period	3.3	2.5
Interest cost on projected benefit obligation	4.8	5.4
Expected return on plan assets	(7.3)	(6.7)
Amortization of prior service cost (credit)	0.2	0.1
Amortization of actuarial gain (loss)	2.9	2.0
Special termination benefits	0.0	1.6
Subtotal	3.9	4.9
Regulatory adjustment	2.8	3.5
Net pension cost (income)	6.7	8.4
Laclede Gas | Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period	2.6	2.7
Interest cost on projected benefit obligation	1.7	2.0
Expected return on plan assets	(2.3)	(2.1)
Amortization of prior service cost (credit)	0.1	0.1
Amortization of actuarial gain (loss)	0.6	1.0
Special termination benefits	0.0	2.6
Subtotal	2.7	6.3
Regulatory adjustment	(0.4)	(3.8)
Net pension cost (income)	2.3	2.5
Alagasco | Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period	1.6	1.4
Interest cost on projected benefit obligation	1.5	1.7
Expected return on plan assets	(1.8)	(2.2)
Amortization of actuarial gain (loss)	0.5	0.0
Subtotal	1.8	0.9
Regulatory adjustment	1.6	1.5
Net pension cost (income)	3.4	2.4
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
Interest cost on projected benefit obligation	0.4	0.5
Expected return on plan assets	(1.1)	(1.3)
Amortization of prior service cost (credit)	(0.1)	0.0
Amortization of actuarial gain (loss)	0.0	(0.1)
Subtotal	(0.7)	(0.8)
Regulatory adjustment	(0.4)	(0.4)
Net pension cost (income)	$ (1.1)	$ (1.2)