Shareholders' Equity - Schedule of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Accumulated Other Comprehensive Income Loss [Line Items]
BALANCE		$ 2,543.0	$ 2,255.4	$ 1,991.3
Other comprehensive (loss) income		(9.9)	(37.7)	2.5
BALANCE		2,522.3	2,543.0	2,255.4
Spire Missouri
Accumulated Other Comprehensive Income Loss [Line Items]
BALANCE		1,339.3	1,259.9	1,171.0
Other comprehensive (loss) income		(0.5)	(0.8)	0.4
BALANCE		1,435.1	1,339.3	1,259.9
Net Unrealized Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income Loss [Line Items]
BALANCE		(29.0)	7.9
Other comprehensive (loss) income		(9.4)	(36.9)
BALANCE		(38.4)	(29.0)	7.9
Net Unrealized Gains (Losses) 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		(2.4)	(1.5)
Other comprehensive (loss) income		(0.5)	(0.9)
BALANCE		(2.9)	(2.4)	(1.5)
Defined Benefit Pension and Other Postretirement Benefit Plans | Spire Missouri
Accumulated Other Comprehensive Income Loss [Line Items]
BALANCE		(2.4)	(1.6)
Other comprehensive (loss) income		(0.5)	(0.8)
BALANCE		(2.9)	(2.4)	(1.6)
Net Unrealized Losses on Available-for-Sale Debt Securities
Accumulated Other Comprehensive Income Loss [Line Items]
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 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]	(31.3)	6.4	3.2
Other comprehensive (loss) income	[1]	(9.9)	(37.7)	2.5
BALANCE	[1]	(41.2)	(31.3)	6.4
AOCI | Spire Missouri
Accumulated Other Comprehensive Income Loss [Line Items]
BALANCE	[1]	(2.4)	(1.6)	(1.7)
Other comprehensive (loss) income	[1]	(0.5)	(0.8)	0.4
BALANCE	[1]	$ (2.9)	$ (2.4)	$ (1.6)

[1] Accumulated other comprehensive income (loss)