STOCKHOLDERS' EQUITY - Schedule of Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	$ 1,573.6	$ 1,508.4	$ 1,046.3
Other comprehensive (loss) income	(2.2)	(0.3)	(0.9)
BALANCE	1,768.2	1,573.6	1,508.4
Laclede Gas
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	1,037.8	1,007.8	973.9
Other comprehensive (loss) income	(0.1)	0.2	0.2
BALANCE	1,068.5	1,037.8	1,007.8
Net Unrealized Gains (Losses) on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	(0.4)	0.2
Other comprehensive (loss) income	(1.9)	(0.6)
BALANCE	(2.3)	(0.4)	0.2
Net Unrealized Gains (Losses) on Cash Flow Hedges | Laclede Gas
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	(0.2)	0.0
Other comprehensive (loss) income	0.3	(0.2)
BALANCE	0.1	(0.2)	0.0
Defined Benefit Pension and Other Postretirement Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	(1.5)	(1.9)
Other comprehensive (loss) income	(0.3)	0.4
BALANCE	(1.8)	(1.5)	(1.9)
Defined Benefit Pension and Other Postretirement Benefit Plans | Laclede Gas
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	(1.5)	(1.9)
Other comprehensive (loss) income	(0.3)	0.4
BALANCE	(1.8)	(1.5)	(1.9)
Net Unrealized Losses on Available for Sale Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	(0.1)	0.0
Other comprehensive (loss) income	0.0	(0.1)
BALANCE	(0.1)	(0.1)	0.0
Net Unrealized Losses on Available for Sale Securities | Laclede Gas
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	0.0	0.0
Other comprehensive (loss) income	(0.1)	0.0
BALANCE	(0.1)	0.0	0.0
AOCI
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	(2.0)	(1.7)	(0.8)
Other comprehensive (loss) income	(2.2)	(0.3)	(0.9)
BALANCE	(4.2)	(2.0)	(1.7)
AOCI | Laclede Gas
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	(1.7)	(1.9)	(2.1)
Other comprehensive (loss) income	(0.1)	0.2	0.2
BALANCE	$ (1.8)	$ (1.7)	$ (1.9)