PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Changes in Plan Assets and Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amortization of actuarial loss	$ (0.4)	$ 0.3	$ (0.4)
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial loss	14.1	46.8	48.3
Amortization of actuarial loss	(12.5)	(8.0)	(7.5)
Acceleration of loss recognized due to settlement	(18.2)	(3.3)	(19.6)
Current year service cost	0.0	5.0	0.0
Current year prior year service cost	(20.7)	0.0	0.0
Amortization of prior service cost	(1.0)	(0.4)	(0.5)
Subtotal	(38.3)	40.1	20.7
Regulatory adjustment	38.0	(39.8)	(21.2)
Total recognized in OCI	(0.3)	0.3	(0.5)
Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial loss	(34.1)	0.8	(8.5)
Amortization of actuarial loss	(2.5)	(3.6)	(5.1)
Current year prior year service cost	(1.4)	(1.8)	(4.9)
Amortization of prior service cost	0.0	(0.3)	(0.8)
Subtotal	(38.0)	(4.9)	(19.3)
Regulatory adjustment	38.0	4.9	19.3
Total recognized in OCI	0.0	0.0	0.0
Spire Missouri
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amortization of actuarial loss	(0.3)	0.3	(0.4)
Spire Missouri | Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial loss	14.8	21.6	26.0
Amortization of actuarial loss	(10.7)	(7.9)	(7.5)
Acceleration of loss recognized due to settlement	(13.5)	0.0	(18.0)
Current year service cost	0.0	5.0	0.0
Current year prior year service cost	0.0	0.0	0.0
Amortization of prior service cost	(1.0)	(0.4)	(0.5)
Subtotal	(10.4)	18.3	0.0
Regulatory adjustment	10.1	(18.0)	(0.5)
Total recognized in OCI	(0.3)	0.3	(0.5)
Spire Missouri | Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial loss	(28.5)	1.4	(2.4)
Amortization of actuarial loss	(2.6)	(3.8)	(5.1)
Current year prior year service cost	0.0	0.0	(4.9)
Amortization of prior service cost	(0.2)	(0.3)	(0.8)
Subtotal	(31.3)	(2.7)	(13.2)
Regulatory adjustment	31.3	2.7	13.2
Total recognized in OCI	0.0	0.0	0.0
Spire Alabama | Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial loss	3.3	25.2	22.3
Amortization of actuarial loss	(1.8)	(0.1)	0.0
Acceleration of loss recognized due to settlement	(4.5)	(3.3)	(1.6)
Current year service cost	0.0	0.0	0.0
Current year prior year service cost	(20.7)	0.0	0.0
Amortization of prior service cost	0.0	0.0	0.0
Subtotal	(23.7)	21.8	20.7
Regulatory adjustment	23.7	(21.8)	(20.7)
Total recognized in OCI	0.0	0.0	0.0
Spire Alabama | Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial loss	(4.5)	(0.6)	(6.1)
Amortization of actuarial loss	0.1	0.2	0.0
Current year prior year service cost	(1.4)	(1.8)	0.0
Amortization of prior service cost	0.2	0.0	0.0
Subtotal	(5.6)	(2.2)	(6.1)
Regulatory adjustment	5.6	2.2	6.1
Total recognized in OCI	$ 0.0	$ 0.0	$ 0.0