PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Assumed Medical Cost Trend Rates and Effect of 1% Changed (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Rate to which the medical cost trend rate is assumed to decline (the ultimate medical cost trend rate)	5.00%	5.00%
Year the rate reaches the ultimate trend	2023	2023
Assumed 1% change in the assumed medical cost trend rate [Abstract]
Effect of 1% increase on net periodic postretirement benefit cost	$ 1.7
Effect of 1% decrease on net periodic postretirement benefit cost	(1.4)
Effect of 1% increase on accumulated postretirement benefit obligation	10.0
Effect of 1% decrease on accumulated postretirement benefit obligation	$ (9.2)
Spire Missouri
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Medical cost trend assumed for next year	7.25%	7.50%
Assumed 1% change in the assumed medical cost trend rate [Abstract]
Effect of 1% increase on net periodic postretirement benefit cost	$ 1.6
Effect of 1% decrease on net periodic postretirement benefit cost	(1.3)
Effect of 1% increase on accumulated postretirement benefit obligation	8.0
Effect of 1% decrease on accumulated postretirement benefit obligation	$ (7.4)
Spire Alabama
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Medical cost trend assumed for next year	7.25%	7.50%
Assumed 1% change in the assumed medical cost trend rate [Abstract]
Effect of 1% increase on net periodic postretirement benefit cost	$ 0.1
Effect of 1% decrease on net periodic postretirement benefit cost	(0.1)
Effect of 1% increase on accumulated postretirement benefit obligation	1.4
Effect of 1% decrease on accumulated postretirement benefit obligation	$ (1.3)