Shareholders' Equity - Schedule of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income Loss [Line Items]
BALANCE		$ 2,255.4	$ 1,991.3	$ 1,768.2
Other comprehensive income (loss)		(37.7)	2.5	7.4
BALANCE		2,543.0	2,255.4	1,991.3
Spire Missouri
Accumulated Other Comprehensive Income Loss [Line Items]
BALANCE		1,259.9	1,171.0	1,068.5
Other comprehensive income (loss)		(0.8)	0.4	0.1
BALANCE		1,339.3	1,259.9	1,171.0
Net Unrealized Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income Loss [Line Items]
BALANCE		7.9	4.9
Other comprehensive income (loss)		(36.9)	2.0
Reclassification of certain income tax effects			1.0
BALANCE		(29.0)	7.9	4.9
Net Unrealized Gains (Losses) on Cash Flow Hedges | Spire Missouri
Accumulated Other Comprehensive Income Loss [Line Items]
BALANCE		0.0	0.1
Other comprehensive income (loss)		0.0	(0.1)
Reclassification of certain income tax effects			0.0
BALANCE		0.0	0.0	0.1
Defined Benefit Pension and Other Postretirement Benefit Plans
Accumulated Other Comprehensive Income Loss [Line Items]
BALANCE		(1.5)	(1.5)
Other comprehensive income (loss)		(0.9)	0.3
Reclassification of certain income tax effects			(0.3)
BALANCE		(2.4)	(1.5)	(1.5)
Defined Benefit Pension and Other Postretirement Benefit Plans | Spire Missouri
Accumulated Other Comprehensive Income Loss [Line Items]
BALANCE		(1.6)	(1.6)
Other comprehensive income (loss)		(0.8)	0.3
Reclassification of certain income tax effects			(0.3)
BALANCE		(2.4)	(1.6)	(1.6)
Net Unrealized Losses on Available-for-Sale Debt Securities
Accumulated Other Comprehensive Income Loss [Line Items]
BALANCE		0.0	(0.2)
Other comprehensive income (loss)		0.1	0.2
Reclassification of certain income tax effects			0.0
BALANCE		0.1	0.0	(0.2)
Net Unrealized Losses on Available-for-Sale Debt Securities | Spire Missouri
Accumulated Other Comprehensive Income Loss [Line Items]
BALANCE		0.0	(0.2)
Other comprehensive income (loss)		0.0	0.2
Reclassification of certain income tax effects			0.0
BALANCE		0.0	0.0	(0.2)
AOCI
Accumulated Other Comprehensive Income Loss [Line Items]
BALANCE	[1]	6.4	3.2	(4.2)
Other comprehensive income (loss)	[1]	(37.7)	2.5	7.4
Reclassification of certain income tax effects	[1]		0.7
BALANCE	[1]	(31.3)	6.4	3.2
AOCI | Spire Missouri
Accumulated Other Comprehensive Income Loss [Line Items]
BALANCE	[1]	(1.6)	(1.7)	(1.8)
Other comprehensive income (loss)	[1]	(0.8)	0.4	0.1
Reclassification of certain income tax effects	[1]		(0.3)
BALANCE	[1]	$ (2.4)	$ (1.6)	$ (1.7)

[1] Accumulated other comprehensive income (loss)