STOCKHOLDERS' EQUITY - Schedule of Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	$ 1,768.2	$ 1,573.6	$ 1,508.4
Other comprehensive loss	7.4	(2.2)	(0.3)
BALANCE	1,991.3	1,768.2	1,573.6
Spire Missouri
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	1,068.5	1,037.8	1,007.8
Other comprehensive loss	0.1	(0.1)	0.2
BALANCE	1,171.0	1,068.5	1,037.8
Net Unrealized Gains (Losses) on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	(2.3)	(0.4)
Other comprehensive loss	7.2	(1.9)
BALANCE	4.9	(2.3)	(0.4)
Net Unrealized Gains (Losses) on Cash Flow Hedges | Spire Missouri
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	0.1	(0.2)
Other comprehensive loss	0.0	0.3
BALANCE	0.1	0.1	(0.2)
Defined Benefit Pension and Other Postretirement Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	(1.8)	(1.5)
Other comprehensive loss	0.3	(0.3)
BALANCE	(1.5)	(1.8)	(1.5)
Defined Benefit Pension and Other Postretirement Benefit Plans | Spire Missouri
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	(1.8)	(1.5)
Other comprehensive loss	0.2	(0.3)
BALANCE	(1.6)	(1.8)	(1.5)
Net Unrealized Losses on Available for Sale Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	(0.1)	(0.1)
Other comprehensive loss	(0.1)	0.0
BALANCE	(0.2)	(0.1)	(0.1)
Net Unrealized Losses on Available for Sale Securities | Spire Missouri
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	(0.1)	0.0
Other comprehensive loss	(0.1)	(0.1)
BALANCE	(0.2)	(0.1)	0.0
AOCI
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	(4.2)	(2.0)	(1.7)
Other comprehensive loss	7.4	(2.2)	(0.3)
BALANCE	3.2	(4.2)	(2.0)
AOCI | Spire Missouri
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	(1.8)	(1.7)	(1.9)
Other comprehensive loss	0.1	(0.1)	0.2
BALANCE	$ (1.7)	$ (1.8)	$ (1.7)