PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Pre-tax Amortization from AOCI (Details) $ in Millions	12 Months Ended
	Sep. 30, 2016 USD ($)
Pension Plans
Pre-tax amounts are expected to be amortized from accumulated other comprehensive income [Abstract]
Amortization of net actuarial loss (gain)	$ 13.5
Amortization of prior service cost	1.0
Subtotal	14.5
Regulatory adjustment	(14.2)
Total	0.3
Postretirement Plans
Pre-tax amounts are expected to be amortized from accumulated other comprehensive income [Abstract]
Amortization of net actuarial loss (gain)	2.4
Amortization of prior service cost	0.1
Subtotal	2.5
Regulatory adjustment	(2.5)
Total	0.0
Laclede Gas | Pension Plans
Pre-tax amounts are expected to be amortized from accumulated other comprehensive income [Abstract]
Amortization of net actuarial loss (gain)	11.6
Amortization of prior service cost	1.0
Subtotal	12.6
Regulatory adjustment	(12.3)
Total	0.3
Laclede Gas | Postretirement Plans
Pre-tax amounts are expected to be amortized from accumulated other comprehensive income [Abstract]
Amortization of net actuarial loss (gain)	2.5
Amortization of prior service cost	0.3
Subtotal	2.8
Regulatory adjustment	(2.8)
Total	0.0
Alagasco | Pension Plans
Pre-tax amounts are expected to be amortized from accumulated other comprehensive income [Abstract]
Amortization of net actuarial loss (gain)	1.9
Amortization of prior service cost	0.0
Subtotal	1.9
Regulatory adjustment	(1.9)
Total	0.0
Alagasco | Postretirement Plans
Pre-tax amounts are expected to be amortized from accumulated other comprehensive income [Abstract]
Amortization of net actuarial loss (gain)	(0.1)
Amortization of prior service cost	(0.2)
Subtotal	(0.3)
Regulatory adjustment	0.3
Total	$ 0.0