Pension Plans and Other Postretirement Benefits - Fair Value Measurements of Plan Assets (Details) - USD ($) $ in Millions	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Pension Plans
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	$ 521.8	$ 499.2	$ 531.6
Pension Plans | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	379.2	349.1	385.9
Pension Plans | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	93.3	101.3	97.9
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	42.5	50.6
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	41.2	35.2
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	0.8	10.4
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	63.9
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	8.4
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	37.4
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	24.8
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	4.1
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	14.0
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	100.7
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	64.9
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	24.2
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	37.3
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	34.7
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	1.8
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	152.1
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	152.1
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	6.6
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	6.6
Pension Plans | International
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	51.6	50.9
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	43.9
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	4.7
Pension Plans | Derivatives and margin receivable (payable)
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.2	(0.8)
Pension Plans | Derivatives and margin receivable (payable) | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.2	(0.8)
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	13.1
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	8.8
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	392.8	380.3
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	286.1	265.7
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	69.8	77.4
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	42.5	50.6
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	41.2	35.2
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	0.8	10.4
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	42.7
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	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	28.8
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	20.7
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	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	14.0
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	35.8
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	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	24.2
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	31.7
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	31.7
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	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	152.1
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	152.1
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	3.6
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	3.6
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	43.9
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	43.9
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	0.0
Pension Plans | Quoted Prices in Active Markets (Level 1) | Derivatives and margin receivable (payable)
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.2	(0.8)
Pension Plans | Quoted Prices in Active Markets (Level 1) | Derivatives and margin receivable (payable) | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.2	(0.8)
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	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	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	129.0	118.9
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	93.1	83.4
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	23.5	23.9
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 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	21.2
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	8.4
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	8.6
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	4.1
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	4.1
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	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	64.9
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	64.9
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	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	5.6
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	3.0
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	1.8
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	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	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	3.0
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	3.0
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	7.0
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	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	4.7
Pension Plans | Significant Observable Inputs (Level 2) | Derivatives and margin receivable (payable)
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) | Derivatives and margin receivable (payable) | 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) | 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	13.1
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	8.8
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 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
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	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	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	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	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	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	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	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	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	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	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	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	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	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	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	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	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	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	0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | Derivatives and margin receivable (payable)
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) | Derivatives and margin receivable (payable) | 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) | 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	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	0.0
Other Postretirement Plans
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	279.8	283.5	265.5
Other Postretirement Plans | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	188.9	189.5	174.9
Other Postretirement Plans | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	86.4	89.6	$ 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	5.3	3.7
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	5.2	3.5
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	258.9	264.7
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	183.7	186.0
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	71.8	74.5
Other Postretirement Plans | International
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	15.6	15.1
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	14.6	15.1
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	193.4	193.9
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	188.9	189.5
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	5.3	3.7
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	5.2	3.5
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	187.1	190.2
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	183.7	186.0
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	0.0
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	86.4	89.6
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	86.4	89.6
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	71.8	74.5
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	71.8	74.5
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	14.6	15.1
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	14.6	15.1
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