Pension Plans and Other Postretirement Benefits - Fair Value Measurements of Plan Assets (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Pension Plans
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	$ 473.1	$ 521.8	$ 499.2
Pension Plans | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	336.2	379.2	349.1
Pension Plans | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	88.6	93.3	101.3
Pension Plans | Cash and cash equivalents
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	11.5	42.5
Pension Plans | Cash and cash equivalents | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	9.5	41.2
Pension Plans | Cash and cash equivalents | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.9	0.8
Pension Plans | Equity funds - global (including U.S.)
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	227.0
Pension Plans | Equity funds - global (including U.S.) | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	159.9
Pension Plans | Equity funds - global (including U.S.) | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	43.6
Pension Plans | Equity index funds - global (including U.S.)
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	32.6
Pension Plans | Equity index funds - global (including U.S.) | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	22.8
Pension Plans | Equity index funds - global (including U.S.) | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	6.4
Pension Plans | Equity mutual funds - U.S.
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		57.5
Pension Plans | Equity mutual funds - U.S. | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		9.1
Pension Plans | Equity mutual funds - U.S. | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		31.7
Pension Plans | Equity mutual funds - international
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		38.7
Pension Plans | Equity mutual funds - international | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		3.3
Pension Plans | Equity mutual funds - international | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		23.2
Pension Plans | U.S. bond funds
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	48.9
Pension Plans | U.S. bond funds | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	31.0
Pension Plans | U.S. bond funds | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	11.7
Pension Plans | U.S. government index funds
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	78.3
Pension Plans | U.S. government index funds | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	59.6
Pension Plans | U.S. government index funds | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	12.1
Pension Plans | Global funds (including U.S.)
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	74.8
Pension Plans | Global funds (including U.S.) | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	53.4
Pension Plans | Global funds (including U.S.) | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	13.9
Pension Plans | Derivatives and margin receivable
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.2
Pension Plans | Derivatives and margin receivable | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.2
Pension Plans | U.S. bond mutual funds
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		108.7
Pension Plans | U.S. bond mutual funds | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		73.9
Pension Plans | U.S. bond mutual funds | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		22.8
Pension Plans | U.S. government
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		62.4
Pension Plans | U.S. government | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		59.7
Pension Plans | U.S. government | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		1.7
Pension Plans | U.S. corporate
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		143.7
Pension Plans | U.S. corporate | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		143.7
Pension Plans | U.S. municipal
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		4.0
Pension Plans | U.S. municipal | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		4.0
Pension Plans | International
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		51.6
Pension Plans | International | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		44.1
Pension Plans | International | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		4.9
Pension Plans | 103-12 Direct Filing Entities
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		12.5
Pension Plans | 103-12 Direct Filing Entities | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		8.2
Pension Plans | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	202.7	392.8
Pension Plans | Quoted Prices in Active Markets (Level 1) | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	145.0	286.1
Pension Plans | Quoted Prices in Active Markets (Level 1) | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	37.2	69.8
Pension Plans | Quoted Prices in Active Markets (Level 1) | Cash and cash equivalents
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	11.5	42.5
Pension Plans | Quoted Prices in Active Markets (Level 1) | Cash and cash equivalents | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	9.5	41.2
Pension Plans | Quoted Prices in Active Markets (Level 1) | Cash and cash equivalents | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.9	0.8
Pension Plans | Quoted Prices in Active Markets (Level 1) | Equity funds - global (including U.S.)
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	31.4
Pension Plans | Quoted Prices in Active Markets (Level 1) | Equity funds - global (including U.S.) | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	22.1
Pension Plans | Quoted Prices in Active Markets (Level 1) | Equity funds - global (including U.S.) | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	6.1
Pension Plans | Quoted Prices in Active Markets (Level 1) | Equity index funds - global (including U.S.)
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	32.6
Pension Plans | Quoted Prices in Active Markets (Level 1) | Equity index funds - global (including U.S.) | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	22.8
Pension Plans | Quoted Prices in Active Markets (Level 1) | Equity index funds - global (including U.S.) | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	6.4
Pension Plans | Quoted Prices in Active Markets (Level 1) | Equity mutual funds - U.S.
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		40.5
Pension Plans | Quoted Prices in Active Markets (Level 1) | Equity mutual funds - U.S. | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Quoted Prices in Active Markets (Level 1) | Equity mutual funds - U.S. | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		26.5
Pension Plans | Quoted Prices in Active Markets (Level 1) | Equity mutual funds - international
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		30.1
Pension Plans | Quoted Prices in Active Markets (Level 1) | Equity mutual funds - international | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Quoted Prices in Active Markets (Level 1) | Equity mutual funds - international | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		19.7
Pension Plans | Quoted Prices in Active Markets (Level 1) | U.S. bond funds
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	48.9
Pension Plans | Quoted Prices in Active Markets (Level 1) | U.S. bond funds | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	31.0
Pension Plans | Quoted Prices in Active Markets (Level 1) | U.S. bond funds | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	11.7
Pension Plans | Quoted Prices in Active Markets (Level 1) | U.S. government index funds
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	78.3
Pension Plans | Quoted Prices in Active Markets (Level 1) | U.S. government index funds | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	59.6
Pension Plans | Quoted Prices in Active Markets (Level 1) | U.S. government index funds | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	12.1
Pension Plans | Quoted Prices in Active Markets (Level 1) | Global funds (including U.S.)
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Pension Plans | Quoted Prices in Active Markets (Level 1) | Global funds (including U.S.) | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Pension Plans | Quoted Prices in Active Markets (Level 1) | Global funds (including U.S.) | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Pension Plans | Quoted Prices in Active Markets (Level 1) | Derivatives and margin receivable
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.2
Pension Plans | Quoted Prices in Active Markets (Level 1) | Derivatives and margin receivable | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.2
Pension Plans | Quoted Prices in Active Markets (Level 1) | U.S. bond mutual funds
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		34.8
Pension Plans | Quoted Prices in Active Markets (Level 1) | U.S. bond mutual funds | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Quoted Prices in Active Markets (Level 1) | U.S. bond mutual funds | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		22.8
Pension Plans | Quoted Prices in Active Markets (Level 1) | U.S. government
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		52.9
Pension Plans | Quoted Prices in Active Markets (Level 1) | U.S. government | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		52.9
Pension Plans | Quoted Prices in Active Markets (Level 1) | U.S. government | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Quoted Prices in Active Markets (Level 1) | U.S. corporate
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		143.7
Pension Plans | Quoted Prices in Active Markets (Level 1) | U.S. corporate | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		143.7
Pension Plans | Quoted Prices in Active Markets (Level 1) | U.S. municipal
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		4.0
Pension Plans | Quoted Prices in Active Markets (Level 1) | U.S. municipal | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		4.0
Pension Plans | Quoted Prices in Active Markets (Level 1) | International
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		44.1
Pension Plans | Quoted Prices in Active Markets (Level 1) | International | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		44.1
Pension Plans | Quoted Prices in Active Markets (Level 1) | International | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Quoted Prices in Active Markets (Level 1) | 103-12 Direct Filing Entities
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Quoted Prices in Active Markets (Level 1) | 103-12 Direct Filing Entities | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Observable Inputs (Level 2)
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	270.4	129.0
Pension Plans | Significant Observable Inputs (Level 2) | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	191.2	93.1
Pension Plans | Significant Observable Inputs (Level 2) | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	51.4	23.5
Pension Plans | Significant Observable Inputs (Level 2) | Cash and cash equivalents
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Pension Plans | Significant Observable Inputs (Level 2) | Cash and cash equivalents | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Pension Plans | Significant Observable Inputs (Level 2) | Cash and cash equivalents | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Pension Plans | Significant Observable Inputs (Level 2) | Equity funds - global (including U.S.)
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	195.6
Pension Plans | Significant Observable Inputs (Level 2) | Equity funds - global (including U.S.) | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	137.8
Pension Plans | Significant Observable Inputs (Level 2) | Equity funds - global (including U.S.) | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	37.5
Pension Plans | Significant Observable Inputs (Level 2) | Equity index funds - global (including U.S.)
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Pension Plans | Significant Observable Inputs (Level 2) | Equity index funds - global (including U.S.) | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Pension Plans | Significant Observable Inputs (Level 2) | Equity index funds - global (including U.S.) | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Pension Plans | Significant Observable Inputs (Level 2) | Equity mutual funds - U.S.
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		17.0
Pension Plans | Significant Observable Inputs (Level 2) | Equity mutual funds - U.S. | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		9.1
Pension Plans | Significant Observable Inputs (Level 2) | Equity mutual funds - U.S. | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		5.2
Pension Plans | Significant Observable Inputs (Level 2) | Equity mutual funds - international
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		8.6
Pension Plans | Significant Observable Inputs (Level 2) | Equity mutual funds - international | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		3.3
Pension Plans | Significant Observable Inputs (Level 2) | Equity mutual funds - international | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		3.5
Pension Plans | Significant Observable Inputs (Level 2) | U.S. bond funds
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Pension Plans | Significant Observable Inputs (Level 2) | U.S. bond funds | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Pension Plans | Significant Observable Inputs (Level 2) | U.S. bond funds | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Pension Plans | Significant Observable Inputs (Level 2) | U.S. government index funds
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Pension Plans | Significant Observable Inputs (Level 2) | U.S. government index funds | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Pension Plans | Significant Observable Inputs (Level 2) | U.S. government index funds | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Pension Plans | Significant Observable Inputs (Level 2) | Global funds (including U.S.)
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	74.8
Pension Plans | Significant Observable Inputs (Level 2) | Global funds (including U.S.) | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	53.4
Pension Plans | Significant Observable Inputs (Level 2) | Global funds (including U.S.) | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	13.9
Pension Plans | Significant Observable Inputs (Level 2) | Derivatives and margin receivable
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Observable Inputs (Level 2) | Derivatives and margin receivable | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Observable Inputs (Level 2) | U.S. bond mutual funds
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		73.9
Pension Plans | Significant Observable Inputs (Level 2) | U.S. bond mutual funds | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		73.9
Pension Plans | Significant Observable Inputs (Level 2) | U.S. bond mutual funds | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Observable Inputs (Level 2) | U.S. government
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		9.5
Pension Plans | Significant Observable Inputs (Level 2) | U.S. government | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		6.8
Pension Plans | Significant Observable Inputs (Level 2) | U.S. government | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		1.7
Pension Plans | Significant Observable Inputs (Level 2) | U.S. corporate
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Observable Inputs (Level 2) | U.S. corporate | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Observable Inputs (Level 2) | U.S. municipal
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Observable Inputs (Level 2) | U.S. municipal | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Observable Inputs (Level 2) | International
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		7.5
Pension Plans | Significant Observable Inputs (Level 2) | International | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Observable Inputs (Level 2) | International | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		4.9
Pension Plans | Significant Observable Inputs (Level 2) | 103-12 Direct Filing Entities
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		12.5
Pension Plans | Significant Observable Inputs (Level 2) | 103-12 Direct Filing Entities | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		8.2
Pension Plans | Significant Unobservable Inputs (Level 3)
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | Cash and cash equivalents
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | Cash and cash equivalents | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | Cash and cash equivalents | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | Equity funds - global (including U.S.)
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | Equity funds - global (including U.S.) | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | Equity funds - global (including U.S.) | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | Equity index funds - global (including U.S.)
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | Equity index funds - global (including U.S.) | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | Equity index funds - global (including U.S.) | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | Equity mutual funds - U.S.
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | Equity mutual funds - U.S. | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | Equity mutual funds - U.S. | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | Equity mutual funds - international
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | Equity mutual funds - international | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | Equity mutual funds - international | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | U.S. bond funds
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | U.S. bond funds | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | U.S. bond funds | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | U.S. government index funds
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | U.S. government index funds | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | U.S. government index funds | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | Global funds (including U.S.)
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | Global funds (including U.S.) | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | Global funds (including U.S.) | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | Derivatives and margin receivable
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | Derivatives and margin receivable | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | U.S. bond mutual funds
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | U.S. bond mutual funds | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | U.S. bond mutual funds | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | U.S. government
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | U.S. government | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | U.S. government | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | U.S. corporate
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | U.S. corporate | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | U.S. municipal
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | U.S. municipal | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | International
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | International | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | International | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | 103-12 Direct Filing Entities
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | 103-12 Direct Filing Entities | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Other Postretirement Plans
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	291.0	279.8	283.5
Other Postretirement Plans | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	199.2	188.9	189.5
Other Postretirement Plans | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	87.0	86.4	$ 89.6
Other Postretirement Plans | Cash and cash equivalents
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	3.6	5.3
Other Postretirement Plans | Cash and cash equivalents | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	3.0	5.2
Other Postretirement Plans | Equity mutual funds - U.S.
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	271.6	258.9
Other Postretirement Plans | Equity mutual funds - U.S. | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	196.2	183.7
Other Postretirement Plans | Equity mutual funds - U.S. | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	72.2	71.8
Other Postretirement Plans | International
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	15.8	15.6
Other Postretirement Plans | International | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	14.8	14.6
Other Postretirement Plans | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	204.0	193.4
Other Postretirement Plans | Quoted Prices in Active Markets (Level 1) | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	199.2	188.9
Other Postretirement Plans | Quoted Prices in Active Markets (Level 1) | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Other Postretirement Plans | Quoted Prices in Active Markets (Level 1) | Cash and cash equivalents
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	3.6	5.3
Other Postretirement Plans | Quoted Prices in Active Markets (Level 1) | Cash and cash equivalents | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	3.0	5.2
Other Postretirement Plans | Quoted Prices in Active Markets (Level 1) | Equity mutual funds - U.S.
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	199.4	187.1
Other Postretirement Plans | Quoted Prices in Active Markets (Level 1) | Equity mutual funds - U.S. | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	196.2	183.7
Other Postretirement Plans | Quoted Prices in Active Markets (Level 1) | Equity mutual funds - U.S. | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Other Postretirement Plans | Quoted Prices in Active Markets (Level 1) | International
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	1.0	1.0
Other Postretirement Plans | Quoted Prices in Active Markets (Level 1) | International | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Other Postretirement Plans | Significant Observable Inputs (Level 2)
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	87.0	86.4
Other Postretirement Plans | Significant Observable Inputs (Level 2) | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Other Postretirement Plans | Significant Observable Inputs (Level 2) | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	87.0	86.4
Other Postretirement Plans | Significant Observable Inputs (Level 2) | Cash and cash equivalents
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Other Postretirement Plans | Significant Observable Inputs (Level 2) | Cash and cash equivalents | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Other Postretirement Plans | Significant Observable Inputs (Level 2) | Equity mutual funds - U.S.
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	72.2	71.8
Other Postretirement Plans | Significant Observable Inputs (Level 2) | Equity mutual funds - U.S. | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Other Postretirement Plans | Significant Observable Inputs (Level 2) | Equity mutual funds - U.S. | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	72.2	71.8
Other Postretirement Plans | Significant Observable Inputs (Level 2) | International
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	14.8	14.6
Other Postretirement Plans | Significant Observable Inputs (Level 2) | International | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	14.8	14.6
Other Postretirement Plans | Significant Unobservable Inputs (Level 3)
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Other Postretirement Plans | Significant Unobservable Inputs (Level 3) | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Other Postretirement Plans | Significant Unobservable Inputs (Level 3) | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Other Postretirement Plans | Significant Unobservable Inputs (Level 3) | Cash and cash equivalents
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Other Postretirement Plans | Significant Unobservable Inputs (Level 3) | Cash and cash equivalents | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Other Postretirement Plans | Significant Unobservable Inputs (Level 3) | Equity mutual funds - U.S.
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Other Postretirement Plans | Significant Unobservable Inputs (Level 3) | Equity mutual funds - U.S. | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Other Postretirement Plans | Significant Unobservable Inputs (Level 3) | Equity mutual funds - U.S. | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Other Postretirement Plans | Significant Unobservable Inputs (Level 3) | International
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Other Postretirement Plans | Significant Unobservable Inputs (Level 3) | International | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	$ 0.0	$ 0.0