PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Net Periodic Cost (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2017	Mar. 31, 2016
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period	$ 5.2	$ 3.8		$ 10.5	$ 7.7
Interest cost on projected benefit obligation	6.9	7.0		13.8	14.1
Expected return on plan assets	(9.7)	(8.8)		(19.6)	(17.7)
Amortization of prior service cost (credit)	0.3	0.1		0.5	0.2
Amortization of actuarial gain (loss)	3.2	2.0		6.6	4.0
Loss (gain) on lump-sum settlements	11.9	(2.2)		11.9	(2.2)
Special termination benefits	0.0	0.0		0.0	1.6
Subtotal	17.8	1.9		23.7	7.7
Regulatory adjustment	(7.4)	6.1		(2.8)	11.1
Net pension cost (income)	10.4	8.0		20.9	18.8
Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period	2.7	2.7		5.5	5.5
Interest cost on projected benefit obligation	2.2	2.5		4.3	5.0
Expected return on plan assets	(3.4)	(3.4)		(6.8)	(6.8)
Amortization of prior service cost (credit)	0.0	0.0		0.0	0.1
Amortization of actuarial gain (loss)	0.6	0.9		1.2	1.8
Special termination benefits	0.0	0.0	$ 2.6	0.0	2.6
Subtotal	2.1	2.7		4.2	8.2
Regulatory adjustment	(0.8)	(1.7)		(1.6)	(5.9)
Net pension cost (income)	1.3	1.0		2.6	2.3
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		6.6	5.0
Interest cost on projected benefit obligation	4.9	5.4		9.7	10.8
Expected return on plan assets	(7.1)	(6.7)		(14.4)	(13.4)
Amortization of prior service cost (credit)	0.3	0.1		0.5	0.2
Amortization of actuarial gain (loss)	2.8	2.0		5.7	4.0
Loss (gain) on lump-sum settlements	11.5	0.0		11.5	0.0
Special termination benefits	0.0	0.0	$ 1.6	0.0	1.6
Subtotal	15.7	3.3		19.6	8.2
Regulatory adjustment	(8.6)	2.3		(5.8)	5.8
Net pension cost (income)	7.1	5.6		13.8	14.0
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	2.6		5.2	5.3
Interest cost on projected benefit obligation	1.7	2.0		3.4	4.0
Expected return on plan assets	(2.2)	(2.2)		(4.5)	(4.3)
Amortization of prior service cost (credit)	0.0	0.0		0.1	0.1
Amortization of actuarial gain (loss)	0.7	0.9		1.3	1.9
Special termination benefits	0.0	0.0		0.0	2.6
Subtotal	2.8	3.3		5.5	9.6
Regulatory adjustment	(0.3)	(1.2)		(0.7)	(5.0)
Net pension cost (income)	2.5	2.1		4.8	4.6
Alagasco | Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period	1.5	1.3		3.1	2.7
Interest cost on projected benefit obligation	1.5	1.6		3.0	3.3
Expected return on plan assets	(1.8)	(2.1)		(3.6)	(4.3)
Amortization of actuarial gain (loss)	0.4	0.0		0.9	0.0
Loss (gain) on lump-sum settlements	0.4	(2.2)		0.4	(2.2)
Subtotal	2.0	(1.4)		3.8	(0.5)
Regulatory adjustment	1.1	3.8		2.7	5.3
Net pension cost (income)	3.1	2.4		6.5	4.8
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	0.2
Interest cost on projected benefit obligation	0.4	0.5		0.8	1.0
Expected return on plan assets	(1.1)	(1.2)		(2.2)	(2.5)
Amortization of prior service cost (credit)	0.0	0.0		(0.1)	0.0
Amortization of actuarial gain (loss)	(0.1)	0.0		(0.1)	(0.1)
Subtotal	(0.7)	(0.6)		(1.4)	(1.4)
Regulatory adjustment	(0.5)	(0.5)		(0.9)	(0.9)
Net pension cost (income)	$ (1.2)	$ (1.1)		$ (2.3)	$ (2.3)