PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Narrative (Details)	3 Months Ended		6 Months Ended
	Mar. 31, 2018 USD ($) plan	Mar. 31, 2017 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 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
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Loss on lump-sum settlements	$ 9,400,000	$ 11,900,000	$ 9,400,000	$ 11,900,000
Regulatory adjustment	$ 24,000,000	(7,400,000)	28,300,000	(2,800,000)
Pension Plans | Spire Missouri
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Number of plans | plan	2
Lump-sum payments recognized as settlement	$ 39,500,000	36,300,000
Loss on lump-sum settlements	$ 9,400,000	11,500,000	$ 9,400,000	11,500,000
Discount rate on defined benefit plan	4.00%		4.00%		3.75%
Regulatory adjustment	$ 21,900,000	(8,600,000)	$ 24,300,000	(5,800,000)
Contributions made by employer			13,500,000
Anticipated contributions to pension plans for qualified trust	22,400,000		22,400,000
Pension Plans | Spire Alabama
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Lump-sum payments recognized as settlement		1,900,000
Loss on lump-sum settlements	0	400,000	0	400,000
Regulatory adjustment	$ 1,900,000	1,100,000	3,600,000	2,700,000
Contributions made by employer			$ 0
Other Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate on defined benefit plan	4.00%		4.00%		3.70%
Other Pension Plans | Spire Missouri
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	$ 500,000		500,000
Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory adjustment	0	(800,000)	100,000	(1,600,000)
Contributions made by employer			1,700,000
Postretirement Plans | Spire Missouri
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory adjustment	500,000	(300,000)	1,000,000	(700,000)
Anticipated contributions to pension plans for qualified trust	5,200,000		5,200,000
Anticipated payment directly to participants in postretirement plan			800,000
Postretirement Plans | Spire Alabama
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory adjustment	(500,000)	$ (500,000)	(900,000)	$ (900,000)
Contributions made by employer			0
Anticipated contributions to pension plans for qualified trust	0		$ 0
MoPSC | ISRS surcharge | Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory adjustment	$ 28,800,000