PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Changes in Plan Assets and Benefit Obligations (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amortization of actuarial loss		$ 0.3	$ (0.4)	$ (0.5)
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial loss		46.8	48.3	15.7
Amortization of actuarial loss		(8.0)	(7.5)	(7.1)
Acceleration of loss recognized due to settlement		(3.3)	(19.6)	(1.5)
Current year prior service credit		5.0	0.0	0.0
Amortization of prior service cost		(0.4)	(0.5)	(0.5)
Subtotal		40.1	20.7	6.6
Regulatory adjustment		(39.8)	(21.2)	(6.1)
Total recognized in OCI		0.3	(0.5)	0.5
Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial loss		0.8	(8.5)	(3.1)
Amortization of actuarial loss		(3.6)	(5.1)	(6.0)
Current year prior service credit		(1.8)	(4.9)	0.0
Amortization of prior service cost		(0.3)	(0.8)	2.5
Subtotal		(4.9)	(19.3)	(6.6)
Regulatory adjustment		4.9	19.3	6.6
Total recognized in OCI		0.0	0.0	0.0
Laclede Gas
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amortization of actuarial loss		0.3	(0.4)	(0.4)
Laclede Gas | Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial loss		21.6	26.0	14.2
Amortization of actuarial loss		(7.9)	(7.5)	(7.1)
Acceleration of loss recognized due to settlement		0.0	(18.0)	(1.5)
Current year prior service credit		5.0	0.0	0.0
Amortization of prior service cost		(0.4)	(0.5)	(0.5)
Subtotal		18.3	0.0	5.1
Regulatory adjustment		(18.0)	(0.5)	(4.7)
Total recognized in OCI		0.3	(0.5)	0.4
Laclede Gas | Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial loss		1.4	(2.4)	(4.2)
Amortization of actuarial loss		(3.8)	(5.1)	(6.0)
Current year prior service credit		0.0	(4.9)	0.0
Amortization of prior service cost		(0.3)	(0.8)	2.5
Subtotal		(2.7)	(13.2)	(7.7)
Regulatory adjustment		2.7	13.2	7.7
Total recognized in OCI		0.0	0.0	$ 0.0
Alagasco | Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial loss	$ 1.5	25.2	22.3
Amortization of actuarial loss	0.0	(0.1)	0.0
Acceleration of loss recognized due to settlement	0.0	(3.3)	(1.6)
Current year prior service credit	0.0	0.0	0.0
Amortization of prior service cost	0.0	0.0	0.0
Subtotal	1.5	21.8	20.7
Regulatory adjustment	(1.5)	(21.8)	(20.7)
Total recognized in OCI	0.0	0.0	0.0
Alagasco | Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial loss	1.1	(0.6)	(6.1)
Amortization of actuarial loss	0.0	0.2	0.0
Current year prior service credit	0.0	(1.8)	0.0
Amortization of prior service cost	0.0	0.0	0.0
Subtotal	1.1	(2.2)	(6.1)
Regulatory adjustment	(1.1)	2.2	6.1
Total recognized in OCI	$ 0.0	$ 0.0	$ 0.0