PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Fair Value Measurements of Plan Assets (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	$ 531.6	$ 540.5	$ 448.9
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	37.3	51.2
Pension Plans | Equity mutual funds - domestic
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	67.5	126.1
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	48.6
Pension Plans | US bond mutual funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	102.9	149.0
Pension Plans | US government
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	37.7	45.1
Pension Plans | US corporate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	183.7	137.4
Pension Plans | US municipal
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	4.2	6.3
Pension Plans | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	52.3	25.3
Pension Plans | Derivative instruments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	(2.6)	0.1
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	414.8	383.6
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	37.3	51.2
Pension Plans | Quoted Prices in Active Markets (Level 1) | Equity mutual funds - domestic
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	42.1	99.3
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	37.4
Pension Plans | Quoted Prices in Active Markets (Level 1) | US bond mutual funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	34.4	23.0
Pension Plans | Quoted Prices in Active Markets (Level 1) | US government
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	33.2	42.1
Pension Plans | Quoted Prices in Active Markets (Level 1) | US corporate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	183.7	137.4
Pension Plans | Quoted Prices in Active Markets (Level 1) | US municipal
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	4.2	6.3
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	45.1	25.3
Pension Plans | Quoted Prices in Active Markets (Level 1) | Derivative instruments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	(2.6)	(1.0)
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	116.8	156.8
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 - domestic
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	25.4	26.7
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	11.2
Pension Plans | Significant Observable Inputs (Level 2) | US bond mutual funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	68.5	126.0
Pension Plans | Significant Observable Inputs (Level 2) | US government
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	4.5	3.0
Pension Plans | Significant Observable Inputs (Level 2) | US 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) | US 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 Observable Inputs (Level 2) | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	7.2	0.0
Pension Plans | Significant Observable Inputs (Level 2) | Derivative instruments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	1.1
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	0.1
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 - domestic
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.1
Pension Plans | Significant Unobservable Inputs (Level 3) | Equity mutual funds - international
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Pension Plans | Significant Unobservable Inputs (Level 3) | US 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) | US 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) | US 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) | US 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) | Derivative instruments
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
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	265.5	246.4	223.3
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	4.0	4.8
Postretirement Plans | Equity mutual funds - domestic
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	245.8	226.4
Postretirement Plans | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	15.7	15.2
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	179.1	163.6
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	4.0	4.8
Postretirement Plans | Quoted Prices in Active Markets (Level 1) | Equity mutual funds - domestic
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	174.1	157.9
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	0.9
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	82.8
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 - domestic
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	71.7	68.5
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	14.7	14.3
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 - domestic
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	385.9	395.7	339.9
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	31.7	46.5
Spire Missouri | Pension Plans | Equity mutual funds - domestic
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	11.9	14.9
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	5.7
Spire Missouri | Pension Plans | US bond mutual funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	68.5	120.2
Spire Missouri | Pension Plans | US government
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	37.7	45.1
Spire Missouri | Pension Plans | US corporate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	183.7	137.4
Spire Missouri | Pension Plans | US municipal
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	4.2	6.3
Spire Missouri | Pension Plans | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	45.1	25.2
Spire Missouri | Pension Plans | Derivative instruments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.1
Spire Missouri | Pension Plans | Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	(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	295.3	256.5
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	31.7	46.5
Spire Missouri | Pension Plans | Quoted Prices in Active Markets (Level 1) | Equity mutual funds - domestic
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
Spire Missouri | Pension Plans | Quoted Prices in Active Markets (Level 1) | US 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) | US government
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	33.2	42.1
Spire Missouri | Pension Plans | Quoted Prices in Active Markets (Level 1) | US corporate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	183.7	137.4
Spire Missouri | Pension Plans | Quoted Prices in Active Markets (Level 1) | US municipal
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	4.2	6.3
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	45.1	25.2
Spire Missouri | Pension Plans | Quoted Prices in Active Markets (Level 1) | Derivative instruments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		(1.0)
Spire Missouri | Pension Plans | Quoted Prices in Active Markets (Level 1) | Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	(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	90.6	139.1
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 - domestic
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	11.9	14.8
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	5.7
Spire Missouri | Pension Plans | Significant Observable Inputs (Level 2) | US bond mutual funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	68.5	120.2
Spire Missouri | Pension Plans | Significant Observable Inputs (Level 2) | US government
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	4.5	3.0
Spire Missouri | Pension Plans | Significant Observable Inputs (Level 2) | US 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) | US 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 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) | Derivative instruments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		1.1
Spire Missouri | Pension Plans | Significant Observable Inputs (Level 2) | Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	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	0.1
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 - domestic
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.1
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
Spire Missouri | Pension Plans | Significant Unobservable Inputs (Level 3) | US 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) | US 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) | US 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) | US 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) | Derivative instruments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		0.0
Spire Missouri | Pension Plans | Significant Unobservable Inputs (Level 3) | Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	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	174.9	159.7	143.6
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	3.9	4.6
Spire Missouri | Postretirement Plans | Equity mutual funds - domestic
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	171.0	155.1
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	174.9	159.7
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	3.9	4.6
Spire Missouri | Postretirement Plans | Quoted Prices in Active Markets (Level 1) | Equity mutual funds - domestic
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	171.0	155.1
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 - domestic
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 - domestic
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
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	97.9	100.0	109.0
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	3.4	0.4
Spire Alabama | Pension Plans | Equity mutual funds - domestic
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	37.5	70.9
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	28.9
Spire Alabama | Pension Plans | US bond mutual funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	23.2	28.7
Spire Alabama | Pension Plans | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	4.9
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	80.2	82.4
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	3.4	0.4
Spire Alabama | Pension Plans | Quoted Prices in Active Markets (Level 1) | Equity mutual funds - domestic
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	28.4	59.0
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	25.2
Spire Alabama | Pension Plans | Quoted Prices in Active Markets (Level 1) | US bond mutual funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	23.2	23.0
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
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	17.7	17.6
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 - domestic
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	9.1	11.9
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	3.7
Spire Alabama | Pension Plans | Significant Observable Inputs (Level 2) | US bond mutual funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	5.7
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	4.9
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 - domestic
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
Spire Alabama | Pension Plans | Significant Unobservable Inputs (Level 3) | US 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) | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	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	86.4	82.8	$ 79.7
Spire Alabama | Postretirement Plans | Equity mutual funds - domestic
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	71.7	68.5
Spire Alabama | Postretirement Plans | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	14.7	14.3
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 - domestic
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	86.4	82.8
Spire Alabama | Postretirement Plans | Significant Observable Inputs (Level 2) | Equity mutual funds - domestic
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	71.7	68.5
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	14.7	14.3
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 - domestic
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