PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Fair Value Measurements of Plan Assets (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	$ 499.2	$ 531.6	$ 540.5
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	50.6	37.3
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	63.9	67.5
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	24.8	48.6
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	100.7	102.9
Pension Plans | U.S. government
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	37.3	37.7
Pension Plans | U.S. corporate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	152.1	183.7
Pension Plans | U.S. municipal
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	6.6	4.2
Pension Plans | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	50.9	52.3
Pension Plans | Derivatives and margin (payable)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	(0.8)	(2.6)
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	13.1
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	380.3	414.8
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	50.6	37.3
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	42.7	42.1
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	20.7	37.4
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	35.8	34.4
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	31.7	33.2
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	152.1	183.7
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	3.6	4.2
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	43.9	45.1
Pension Plans | Quoted Prices in Active Markets (Level 1) | Derivatives and margin (payable)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	(0.8)	(2.6)
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 | Significant Observable Inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	118.9	116.8
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) | Equity mutual funds - U.S.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	21.2	25.4
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	4.1	11.2
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	64.9	68.5
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	5.6	4.5
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. municipal
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	3.0	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	7.0	7.2
Pension Plans | Significant Observable Inputs (Level 2) | Derivatives and margin (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) | 103-12 Direct Filing Entities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	13.1
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) | 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) | 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 - 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) | 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. 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. 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. 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) | 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) | Derivatives and margin (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) | 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
Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	283.5	265.5	246.4
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	3.7	4.0
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	264.7	245.8
Postretirement Plans | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	15.1	15.7
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	193.9	179.1
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	3.7	4.0
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	190.2	174.1
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	0.0	1.0
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	89.6	86.4
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
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	74.5	71.7
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	15.1	14.7
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
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
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
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
Spire Missouri | Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	349.1	385.9	395.7
Spire Missouri | 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	35.2	31.7
Spire Missouri | 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	8.4	11.9
Spire Missouri | 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	4.1	5.7
Spire Missouri | 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	64.9	68.5
Spire Missouri | Pension Plans | U.S. government
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	34.7	37.7
Spire Missouri | Pension Plans | U.S. corporate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	152.1	183.7
Spire Missouri | Pension Plans | U.S. municipal
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	6.6	4.2
Spire Missouri | Pension Plans | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	43.9	45.1
Spire Missouri | Pension Plans | Derivatives and margin (payable)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	(0.8)	(2.6)
Spire Missouri | 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	265.7	295.3
Spire Missouri | 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	35.2	31.7
Spire Missouri | 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	0.0	0.0
Spire Missouri | 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	0.0	0.0
Spire Missouri | 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	0.0	0.0
Spire Missouri | 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	31.7	33.2
Spire Missouri | 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	152.1	183.7
Spire Missouri | 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	3.6	4.2
Spire Missouri | 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	43.9	45.1
Spire Missouri | Pension Plans | Quoted Prices in Active Markets (Level 1) | Derivatives and margin (payable)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	(0.8)	(2.6)
Spire Missouri | 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	83.4	90.6
Spire Missouri | 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
Spire Missouri | 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	8.4	11.9
Spire Missouri | 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	4.1	5.7
Spire Missouri | 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	64.9	68.5
Spire Missouri | 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	3.0	4.5
Spire Missouri | 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
Spire Missouri | 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	3.0	0.0
Spire Missouri | 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	0.0	0.0
Spire Missouri | Pension Plans | Significant Observable Inputs (Level 2) | Derivatives and margin (payable)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Spire Missouri | 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
Spire Missouri | 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
Spire Missouri | 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
Spire Missouri | 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
Spire Missouri | 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
Spire Missouri | 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
Spire Missouri | 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
Spire Missouri | 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
Spire Missouri | 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
Spire Missouri | Pension Plans | Significant Unobservable Inputs (Level 3) | Derivatives and margin (payable)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Spire Missouri | Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	189.5	174.9	159.7
Spire Missouri | 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	3.5	3.9
Spire Missouri | 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	186.0	171.0
Spire Missouri | 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	189.5	174.9
Spire Missouri | 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	3.5	3.9
Spire Missouri | 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	186.0	171.0
Spire Missouri | 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	0.0	0.0
Spire Missouri | 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
Spire Missouri | 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	0.0	0.0
Spire Missouri | 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
Spire Missouri | 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
Spire Missouri | 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
Spire Alabama
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	8.8
Spire Alabama | 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	0.0
Spire Alabama | Significant Observable Inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	8.8
Spire Alabama | 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
Spire Alabama | Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	101.3	97.9	100.0
Spire Alabama | 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	10.4	3.4
Spire Alabama | 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	37.4	37.5
Spire Alabama | 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	14.0	28.9
Spire Alabama | 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	24.2	23.2
Spire Alabama | Pension Plans | U.S. government
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	1.8
Spire Alabama | Pension Plans | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	4.7	4.9
Spire Alabama | 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	77.4	80.2
Spire Alabama | 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	10.4	3.4
Spire Alabama | 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	28.8	28.4
Spire Alabama | 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	14.0	25.2
Spire Alabama | 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	24.2	23.2
Spire Alabama | 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	0.0
Spire Alabama | 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	0.0	0.0
Spire Alabama | 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	23.9	17.7
Spire Alabama | 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
Spire Alabama | 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	8.6	9.1
Spire Alabama | 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	0.0	3.7
Spire Alabama | 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	0.0	0.0
Spire Alabama | 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	1.8
Spire Alabama | 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	4.7	4.9
Spire Alabama | 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
Spire Alabama | 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
Spire Alabama | 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
Spire Alabama | 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
Spire Alabama | 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
Spire Alabama | 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
Spire Alabama | 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
Spire Alabama | Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	89.6	86.4	$ 82.8
Spire Alabama | 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	74.5	71.7
Spire Alabama | Postretirement Plans | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	15.1	14.7
Spire Alabama | 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	0.0	0.0
Spire Alabama | 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	0.0	0.0
Spire Alabama | 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	0.0	0.0
Spire Alabama | 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	89.6	86.4
Spire Alabama | 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	74.5	71.7
Spire Alabama | 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	15.1	14.7
Spire Alabama | 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
Spire Alabama | 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
Spire Alabama | 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