Shareholders' Equity - Schedule of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Accumulated Other Comprehensive Income Loss [Line Items]
BALANCE		$ 2,522.3	$ 2,543.0	$ 2,255.4
Other comprehensive (loss) income		44.8	(9.9)	(37.7)
BALANCE		2,658.2	2,522.3	2,543.0
Spire Missouri
Accumulated Other Comprehensive Income Loss [Line Items]
BALANCE		1,435.1	1,339.3	1,259.9
Other comprehensive (loss) income		(1.3)	(0.5)	(0.8)
BALANCE		1,577.9	1,435.1	1,339.3
Net Unrealized Gain (Loss) on Cash Flow Hedges
Accumulated Other Comprehensive Income Loss [Line Items]
BALANCE		(38.4)	(29.0)
Other comprehensive (loss) income		46.3	(9.4)
BALANCE		7.9	(38.4)	(29.0)
Net Unrealized Gain (Loss) on Cash Flow Hedges | Spire Missouri
Accumulated Other Comprehensive Income Loss [Line Items]
BALANCE		0.0	0.0
Other comprehensive (loss) income		0.0	0.0
BALANCE		0.0	0.0	0.0
Defined Benefit Pension and Other Postretirement Benefit Plans
Accumulated Other Comprehensive Income Loss [Line Items]
BALANCE		(2.9)	(2.4)
Other comprehensive (loss) income		(1.3)	(0.5)
BALANCE		(4.2)	(2.9)	(2.4)
Defined Benefit Pension and Other Postretirement Benefit Plans | Spire Missouri
Accumulated Other Comprehensive Income Loss [Line Items]
BALANCE		(2.9)	(2.4)
Other comprehensive (loss) income		(1.3)	(0.5)
BALANCE		(4.2)	(2.9)	(2.4)
Net Unrealized Gain (Loss) on Available-for-Sale Debt Securities
Accumulated Other Comprehensive Income Loss [Line Items]
BALANCE		0.1	0.1
Other comprehensive (loss) income		(0.2)	0.0
BALANCE		(0.1)	0.1	0.1
Net Unrealized Gain (Loss) on Available-for-Sale Debt Securities | Spire Missouri
Accumulated Other Comprehensive Income Loss [Line Items]
BALANCE		0.0	0.0
Other comprehensive (loss) income		0.0	0.0
BALANCE		0.0	0.0	0.0
AOCI
Accumulated Other Comprehensive Income Loss [Line Items]
BALANCE	[1]	(41.2)	(31.3)	6.4
Other comprehensive (loss) income	[1]	44.8	(9.9)	(37.7)
BALANCE	[1]	3.6	(41.2)	(31.3)
AOCI | Spire Missouri
Accumulated Other Comprehensive Income Loss [Line Items]
BALANCE	[1]	(2.9)	(2.4)	(1.6)
Other comprehensive (loss) income	[1]	(1.3)	(0.5)	(0.8)
BALANCE	[1]	$ (4.2)	$ (2.9)	$ (2.4)

[1] Accumulated other comprehensive income (loss)