PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Narrative (Details) - USD ($)	3 Months Ended				9 Months Ended		12 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2017	Mar. 07, 2018	Apr. 11, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Medical insurance available until age is reached after early retirement					65 years
Regulatory asset	$ 791,800,000		$ 985,100,000		$ 791,800,000	$ 985,100,000	$ 966,600,000
Remaining recovery period for regulatory assets for which no return on investment during recovery period is provided (in years)							15 years
Spire Missouri
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory asset	528,900,000		650,600,000		528,900,000	650,600,000	$ 653,400,000
Spire Alabama
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory asset	220,300,000		290,900,000		220,300,000	290,900,000	$ 261,600,000
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Lump-sum payments recognized as settlement	34,500,000	$ 39,500,000
Loss on lump-sum settlements	7,500,000	$ 9,400,000	0		16,900,000	11,900,000
Regulatory adjustment	2,100,000		3,100,000		30,400,000	300,000
Pension Plans | Spire Missouri
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Lump-sum payments recognized as settlement	19,600,000			$ 36,300,000
Loss on lump-sum settlements	$ 5,200,000		0	11,500,000	$ 14,600,000	11,500,000
Discount rate on defined benefit plan	4.20%				4.20%		3.70%
Contributions made by employer					$ 24,900,000
Anticipated contributions to pension plans for qualified trust	$ 11,000,000				11,000,000
Regulatory adjustment	2,400,000		1,300,000		26,700,000	(4,500,000)
Pension Plans | Spire Alabama
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Lump-sum payments recognized as settlement	14,900,000			1,900,000
Loss on lump-sum settlements	$ 2,300,000		0	$ 400,000	$ 2,300,000	400,000
Discount rate on defined benefit plan	4.20%				4.20%		3.65%
Contributions made by employer					$ 0
Regulatory adjustment	$ (600,000)		1,600,000		$ 3,000,000	4,300,000
Other Pension Plans | Spire Missouri
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate on defined benefit plan	4.15%				4.15%		3.75%
Contributions made by employer					$ 0
Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contributions made by employer					3,400,000
Regulatory adjustment	$ 700,000		(800,000)		800,000	(2,400,000)
Postretirement Plans | Spire Missouri
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Anticipated contributions to pension plans for qualified trust	3,500,000				3,500,000
Regulatory adjustment	1,100,000		(400,000)		2,100,000	(1,100,000)
Postretirement Plans | Spire Alabama
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contributions made by employer					0
Anticipated contributions to pension plans for qualified trust	0				0
Regulatory adjustment	(500,000)		$ (500,000)		(1,400,000)	$ (1,400,000)
MoPSC | ISRS surcharge | Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory adjustment	$ 28,800,000
Pension costs | MoPSC | ISRS surcharge | Spire Missouri West
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory asset								$ 5,500,000	$ 9,900,000
Pension costs | MoPSC | ISRS surcharge | Spire Missouri East
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory asset								29,000,000	15,500,000
Postretirement health care costs | MoPSC | ISRS surcharge | Spire Missouri East
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory asset								$ 8,600,000	$ 9,500,000
Pension and postretirement benefit costs | Spire Missouri West
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amortization of regulatory assets not earning a return					$ 26,200,000
Pension and postretirement benefit costs | Spire Missouri East
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Remaining recovery period for regulatory assets for which no return on investment during recovery period is provided (in years)					8 years
Amortization of regulatory assets not earning a return					$ 173,000,000