PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Net Periodic Cost (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost – benefits earned during the period		$ 15.3	$ 17.3	$ 10.2
Interest cost on projected benefit obligation		28.0	29.5	24.5
Expected return on plan assets		(34.9)	(37.4)	(27.2)
Amortization of other comprehensive income		0.0	0.0	0.4
Amortization of prior service cost		0.4	0.5	0.5
Amortization of actuarial loss		8.0	7.5	7.1
Loss on lump-sum settlements		3.3	19.6	1.5
Special termination benefits		1.6	0.0	0.0
Subtotal		21.7	37.0	17.0
Regulatory adjustment		17.8	(2.1)	10.4
Net benefit cost		39.5	34.9	27.4
Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost – benefits earned during the period		10.9	12.8	11.3
Interest cost on projected benefit obligation		10.2	11.2	8.9
Expected return on plan assets		(13.5)	(13.2)	(7.3)
Amortization of other comprehensive income		0.0	0.0	(0.2)
Amortization of prior service cost		0.3	0.8	0.0
Amortization of actuarial loss		3.6	5.1	6.0
Special termination benefits		2.6	0.0	0.0
Subtotal		14.1	16.7	18.7
Regulatory adjustment		(6.6)	(11.0)	(9.6)
Net benefit cost		7.5	5.7	9.1
Laclede Gas | Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost – benefits earned during the period		10.0	11.5	9.7
Interest cost on projected benefit obligation		21.7	23.3	24.0
Expected return on plan assets		(26.7)	(29.2)	(26.5)
Amortization of other comprehensive income		0.0	0.0	0.0
Amortization of prior service cost		0.4	0.5	0.5
Amortization of actuarial loss		7.9	7.5	7.1
Loss on lump-sum settlements		0.0	18.0	1.5
Special termination benefits		1.6	0.0	0.0
Subtotal		14.9	31.6	16.3
Regulatory adjustment		11.7	(5.2)	10.4
Net benefit cost		26.6	26.4	26.7
Laclede Gas | Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost – benefits earned during the period		10.6	12.3	11.2
Interest cost on projected benefit obligation		8.1	8.6	8.7
Expected return on plan assets		(8.5)	(8.1)	(6.8)
Amortization of other comprehensive income		0.0	0.0	0.0
Amortization of prior service cost		0.3	0.8	0.0
Amortization of actuarial loss		3.8	5.1	6.0
Special termination benefits		2.6	0.0	0.0
Subtotal		16.9	18.7	19.1
Regulatory adjustment		(4.8)	(9.2)	(9.6)
Net benefit cost		12.1	9.5	$ 9.5
Alagasco | Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost – benefits earned during the period	$ 5.1	5.3	5.8
Interest cost on projected benefit obligation	4.1	6.3	6.2
Expected return on plan assets	(5.2)	(8.2)	(8.2)
Amortization of other comprehensive income	0.0	0.0	0.0
Amortization of prior service cost	0.1	0.0	0.0
Amortization of actuarial loss	2.2	0.1	0.0
Loss on lump-sum settlements	10.1	3.3	1.6
Special termination benefits	0.0	0.0	0.0
Subtotal	16.4	6.8	5.4
Regulatory adjustment	0.4	6.1	3.1
Net benefit cost	16.8	12.9	8.5
Alagasco | Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost – benefits earned during the period	0.4	0.3	0.5
Interest cost on projected benefit obligation	1.9	2.1	2.6
Expected return on plan assets	(3.6)	(5.0)	(5.1)
Amortization of other comprehensive income	0.0	0.0	0.0
Amortization of prior service cost	0.0	0.0	0.0
Amortization of actuarial loss	(1.0)	(0.2)	0.0
Special termination benefits	0.0	0.0	0.0
Subtotal	(2.3)	(2.8)	(2.0)
Regulatory adjustment	(0.2)	(1.8)	(1.8)
Net benefit cost	$ (2.5)	$ (4.6)	$ (3.8)