SHAREHOLDERS' EQUITY - Schedule of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	$ 1,991.3	$ 1,768.2	$ 1,573.6
Other comprehensive income (loss)	2.5	7.4	(2.2)
Reclassification of certain income tax effects	0.7
BALANCE	2,255.4	1,991.3	1,768.2
Spire Missouri
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	1,171.0	1,068.5	1,037.8
Other comprehensive income (loss)	0.4	0.1	(0.1)
Reclassification of certain income tax effects	(0.3)
BALANCE	1,259.9	1,171.0	1,068.5
Net Unrealized Gains (Losses) on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	4.9	(2.3)
Other comprehensive income (loss)	2.0	7.2
Reclassification of certain income tax effects	1.0
BALANCE	7.9	4.9	(2.3)
Net Unrealized Gains (Losses) on Cash Flow Hedges | Spire Missouri
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	0.1	0.1
Other comprehensive income (loss)	(0.1)	0.0
Reclassification of certain income tax effects	0.0
BALANCE	0.0	0.1	0.1
Defined Benefit Pension and Other Postretirement Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	(1.5)	(1.8)
Other comprehensive income (loss)	0.3	0.3
Reclassification of certain income tax effects	(0.3)
BALANCE	(1.5)	(1.5)	(1.8)
Defined Benefit Pension and Other Postretirement Benefit Plans | Spire Missouri
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	(1.6)	(1.8)
Other comprehensive income (loss)	0.3	0.2
Reclassification of certain income tax effects	(0.3)
BALANCE	(1.6)	(1.6)	(1.8)
Net Unrealized Losses on Available-for-Sale Debt Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	(0.2)	(0.1)
Other comprehensive income (loss)	0.2	(0.1)
Reclassification of certain income tax effects	0.0
BALANCE	0.0	(0.2)	(0.1)
Net Unrealized Losses on Available-for-Sale Debt Securities | Spire Missouri
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	(0.2)	(0.1)
Other comprehensive income (loss)	0.2	(0.1)
Reclassification of certain income tax effects	0.0
BALANCE	0.0	(0.2)	(0.1)
AOCI
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	3.2	(4.2)	(2.0)
Other comprehensive income (loss)	2.5	7.4	(2.2)
Reclassification of certain income tax effects	0.7
BALANCE	6.4	3.2	(4.2)
AOCI | Spire Missouri
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE	(1.7)	(1.8)	(1.7)
Other comprehensive income (loss)	0.4	0.1	(0.1)
Reclassification of certain income tax effects	(0.3)
BALANCE	$ (1.6)	$ (1.7)	$ (1.8)