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, 2013	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial (gain) loss			$ 48.3	$ 15.7		$ 17.0
Amortization of actuarial loss			(7.5)	(7.1)		(10.7)
Acceleration of loss recognized due to settlement			(19.6)	(1.5)		(27.0)
Amortization of prior service cost			(0.5)	(0.5)		(0.5)
Subtotal			20.7	6.6		(21.2)
Regulatory adjustment			(21.2)	(6.1)		21.1
Total recognized in other comprehensive income			(0.5)	0.5		(0.1)
Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial (gain) loss			(8.5)	(3.1)		16.3
Amortization of actuarial loss			(5.1)	(6.0)		(5.3)
Amortization of prior service cost			(0.8)	2.5		0.0
Current year prior service credit			(4.9)	0.0		0.0
Amortization of transition obligation			0.0	0.0		(0.1)
Subtotal			(19.3)	(6.6)		10.9
Regulatory adjustment			19.3	6.6		(10.9)
Total recognized in other comprehensive income			0.0	0.0		0.0
Laclede Gas | Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial (gain) loss			26.0	14.2		17.0
Amortization of actuarial loss			(7.5)	(7.1)		(10.7)
Acceleration of loss recognized due to settlement			(18.0)	(1.5)		(27.0)
Amortization of prior service cost			(0.5)	(0.5)		(0.5)
Subtotal			0.0	5.1		(21.2)
Regulatory adjustment			(0.5)	(4.7)		21.1
Total recognized in other comprehensive income			(0.5)	0.4		(0.1)
Laclede Gas | Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial (gain) loss			(2.4)	(4.2)		16.3
Amortization of actuarial loss			(5.1)	(6.0)		(5.3)
Amortization of prior service cost			(0.8)	2.5		0.0
Current year prior service credit			(4.9)	0.0		0.0
Amortization of transition obligation			0.0	0.0		(0.1)
Subtotal			(13.2)	(7.7)		10.9
Regulatory adjustment			13.2	7.7		(10.9)
Total recognized in other comprehensive income			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 (gain) loss	$ 1.5	$ (14.1)	22.3		$ (14.1)
Amortization of actuarial loss	0.0	(8.9)	0.0		(8.9)
Acceleration of loss recognized due to settlement	0.0	0.0	(1.6)		0.0
Amortization of prior service cost	0.0	(0.3)	0.0		(0.3)
Subtotal	1.5	(23.3)	20.7		(23.3)
Regulatory adjustment	(1.5)	0.0	(20.7)		0.0
Total recognized in other comprehensive income	$ 0.0	$ (23.3)	0.0		$ (23.3)
Alagasco | Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial (gain) loss			(6.1)	1.1		(8.1)
Amortization of actuarial loss			0.0	0.0		0.6
Amortization of transition obligation			0.0	0.0		(0.3)
Subtotal			(6.1)	1.1		(7.8)
Regulatory adjustment			6.1	(1.1)		0.0
Total recognized in other comprehensive income			$ 0.0	$ 0.0		$ (7.8)