Pension Plans and Other Postretirement Benefits - Fair Value Measurements of Plan Assets (Details) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Pension Plans
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	$ 498.9	$ 473.1	$ 521.8
Pension Plans | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	364.0	336.2	379.2
Pension Plans | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	82.8	88.6	93.3
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	10.6	11.5
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	8.3	9.5
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	1.4	0.9
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	201.2	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	148.6	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	32.3	43.6
Pension Plans | Real asset funds
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	87.7
Pension Plans | Real asset funds | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	59.2
Pension Plans | Real asset funds | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	14.6
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 | U.S. bond funds
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	58.4	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	38.4	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	12.3	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	60.1	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	45.6	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	8.9	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	80.9	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	63.9	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	13.3	13.9
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	129.1	202.7
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	92.3	145.0
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	22.6	37.2
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	10.6	11.5
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	8.3	9.5
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	1.4	0.9
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	0.0	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	0.0	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	0.0	6.1
Pension Plans | Quoted Prices in Active Markets (Level 1) | Real asset funds
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) | Real asset 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) | Real asset 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 | 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) | U.S. bond funds
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	58.4	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	38.4	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	12.3	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	60.1	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	45.6	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	8.9	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	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	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	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	369.8	270.4
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	271.7	191.2
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	60.2	51.4
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	201.2	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	148.6	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	32.3	37.5
Pension Plans | Significant Observable Inputs (Level 2) | Real asset funds
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	87.7
Pension Plans | Significant Observable Inputs (Level 2) | Real asset funds | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	59.2
Pension Plans | Significant Observable Inputs (Level 2) | Real asset funds | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	14.6
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) | 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	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	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	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	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	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	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	80.9	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	63.9	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	13.3	13.9
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	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	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	0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | Real asset 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) | Real asset 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) | Real asset 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) | 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) | 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	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	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	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	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	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	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	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	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	0.0
Other Postretirement Plans
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	326.9	291.0	279.8
Other Postretirement Plans | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	221.7	199.2	188.9
Other Postretirement Plans | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	99.4	87.0	$ 86.4
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	3.1	3.6
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	2.4	3.0
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	305.7	271.6
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	219.3	196.2
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	82.5	72.2
Other Postretirement Plans | Debt Security, Corporate, Non-US
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	18.1	15.8
Other Postretirement Plans | Debt Security, Corporate, Non-US | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	16.9	14.8
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	227.5	204.0
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	221.7	199.2
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	3.1	3.6
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	2.4	3.0
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	223.2	199.4
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	219.3	196.2
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) | Debt Security, Corporate, Non-US
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	1.2	1.0
Other Postretirement Plans | Quoted Prices in Active Markets (Level 1) | Debt Security, Corporate, Non-US | 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	99.4	87.0
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	99.4	87.0
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	82.5	72.2
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	82.5	72.2
Other Postretirement Plans | Significant Observable Inputs (Level 2) | Debt Security, Corporate, Non-US
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	16.9	14.8
Other Postretirement Plans | Significant Observable Inputs (Level 2) | Debt Security, Corporate, Non-US | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	16.9	14.8
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) | Debt Security, Corporate, Non-US
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) | Debt Security, Corporate, Non-US | 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