PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Fair Value Measurements of Plan Assets (Details) - USD ($) $ in Millions	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	$ 448.9	$ 506.6		$ 345.4
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	43.6	10.2
Pension Plans | Stock/bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	130.6	138.2
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	9.3	73.6
Pension Plans | US government
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	58.7	64.5
Pension Plans | US corporate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	166.6	164.0
Pension Plans | US municipal
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	5.9	8.2
Pension Plans | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	31.3	35.5
Pension Plans | Derivative instruments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	2.9	(1.0)
Pension Plans | Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		13.4
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	272.2	136.4
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	43.4	8.6
Pension Plans | Quoted Prices in Active Markets (Level 1) | Stock/bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	46.4	54.2
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	73.6
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	58.7	0.0
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	123.7	0.0
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	0.0	0.0
Pension Plans | Quoted Prices in Active Markets (Level 1) | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
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	0.0	0.0
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		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	167.1	360.9
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	0.2	1.6
Pension Plans | Significant Observable Inputs (Level 2) | Stock/bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	74.6	74.7
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	9.3	0.0
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	0.0	64.5
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	42.9	164.0
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	5.9	8.2
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	31.3	35.5
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	2.9	(1.0)
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		13.4
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	9.6	9.3
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) | Stock/bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	9.6	9.3
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
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
Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	223.3	222.5		111.6
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	1.6	2.3
Postretirement Plans | Stock/bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	208.3	213.0
Postretirement Plans | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	13.4	7.2
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	117.1	222.5
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	1.6	2.3
Postretirement Plans | Quoted Prices in Active Markets (Level 1) | Stock/bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	115.5	213.0
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	7.2
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	106.2	0.0
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) | Stock/bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	92.8	0.0
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	13.4	0.0
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) | Stock/bond mutual fund
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
Laclede Gas | Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	339.9	387.4		345.4
Cash collateral	8.3
Derivative liability	5.4
Derivative Assets, fair value measurements		2.9
Cash margin payable		3.9
Laclede Gas | 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	31.8	8.3
Laclede Gas | Pension Plans | Stock/bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	67.7	48.5
Laclede Gas | 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		73.6
Laclede Gas | 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	60.5
Laclede Gas | Pension Plans | US corporate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	166.6	154.5
Laclede Gas | Pension Plans | US municipal
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	5.9	8.2
Laclede Gas | Pension Plans | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	27.3	34.8
Laclede Gas | Pension Plans | Derivative instruments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	2.9	(1.0)
Laclede Gas | 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	193.2	81.9
Laclede Gas | 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	31.8	8.3
Laclede Gas | Pension Plans | Quoted Prices in Active Markets (Level 1) | Stock/bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Laclede Gas | 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		73.6
Laclede Gas | 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	37.7	0.0
Laclede Gas | 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	123.7	0.0
Laclede Gas | 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	0.0	0.0
Laclede Gas | 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
Laclede Gas | 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	0.0	0.0
Laclede Gas | 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	146.6	296.2
Laclede Gas | 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
Laclede Gas | Pension Plans | Significant Observable Inputs (Level 2) | Stock/bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	67.6	39.2
Laclede Gas | 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
Laclede Gas | 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	0.0	60.5
Laclede Gas | 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	42.9	154.5
Laclede Gas | 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	5.9	8.2
Laclede Gas | 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	27.3	34.8
Laclede Gas | 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	2.9	(1.0)
Laclede Gas | 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	0.1	9.3
Laclede Gas | 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
Laclede Gas | Pension Plans | Significant Unobservable Inputs (Level 3) | Stock/bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.1	9.3
Laclede Gas | 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
Laclede Gas | 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
Laclede Gas | 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
Laclede Gas | 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
Laclede Gas | 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
Laclede Gas | 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
Laclede Gas | Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	143.6	137.2		$ 111.6
Laclede Gas | 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	1.6	2.3
Laclede Gas | Postretirement Plans | Stock/bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	142.0	134.9
Laclede Gas | 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	117.1	137.2
Laclede Gas | 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	1.6	2.3
Laclede Gas | Postretirement Plans | Quoted Prices in Active Markets (Level 1) | Stock/bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	115.5	134.9
Laclede Gas | 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	26.5	0.0
Laclede Gas | 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
Laclede Gas | Postretirement Plans | Significant Observable Inputs (Level 2) | Stock/bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	26.5	0.0
Laclede Gas | 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
Laclede Gas | 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
Laclede Gas | Postretirement Plans | Significant Unobservable Inputs (Level 3) | Stock/bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Alagasco | Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	109.0	119.2	$ 219.5
Alagasco | Pension Plans | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	11.8	1.9
Alagasco | Pension Plans | Stock/bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	62.9	89.7
Alagasco | 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	9.3
Alagasco | Pension Plans | US government
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	21.0	4.0
Alagasco | Pension Plans | US corporate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		9.5
Alagasco | Pension Plans | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	4.0	0.7
Alagasco | Pension Plans | Derivative instruments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0
Alagasco | Pension Plans | Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets		13.4
Alagasco | 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	58.0	54.5
Alagasco | Pension Plans | Quoted Prices in Active Markets (Level 1) | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	11.6	0.3
Alagasco | Pension Plans | Quoted Prices in Active Markets (Level 1) | Stock/bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	46.4	54.2
Alagasco | 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
Alagasco | 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	0.0	$ 0.0
Alagasco | 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
Alagasco | 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
Alagasco | 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	0.0
Alagasco | 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		$ 0.0
Alagasco | 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	41.5	64.7
Alagasco | 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	0.2	1.6
Alagasco | Pension Plans | Significant Observable Inputs (Level 2) | Stock/bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	7.0	35.5
Alagasco | 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	9.3
Alagasco | 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	21.0	4.0
Alagasco | 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		9.5
Alagasco | 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	4.0	0.7
Alagasco | 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
Alagasco | 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		13.4
Alagasco | 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	9.5	0.0
Alagasco | 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
Alagasco | Pension Plans | Significant Unobservable Inputs (Level 3) | Stock/bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	9.5	0.0
Alagasco | 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
Alagasco | 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
Alagasco | 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
Alagasco | 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
Alagasco | 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
Alagasco | 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
Alagasco | Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	79.7	85.3	$ 98.6
Alagasco | 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		0.1
Alagasco | Postretirement Plans | Stock/bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	66.3	78.0
Alagasco | Postretirement Plans | International
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	13.4	7.2
Alagasco | 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	50.9
Alagasco | 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		0.1
Alagasco | Postretirement Plans | Quoted Prices in Active Markets (Level 1) | Stock/bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	43.6
Alagasco | 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	7.2
Alagasco | 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	79.7	34.4
Alagasco | 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
Alagasco | Postretirement Plans | Significant Observable Inputs (Level 2) | Stock/bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	66.3	34.4
Alagasco | 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	13.4	0.0
Alagasco | 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
Alagasco | 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
Alagasco | Postretirement Plans | Significant Unobservable Inputs (Level 3) | Stock/bond mutual fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value measurements of plan assets	0.0	0.0
Alagasco | 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