PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Net Periodic Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period	$ 5.2	$ 5.3
Interest cost on projected benefit obligation	6.9	6.9
Expected return on plan assets	(9.7)	(9.9)
Amortization of prior service credit	(0.3)	0.2
Amortization of actuarial gain (loss)	3.1	3.4
Subtotal	5.2	5.9
Regulatory adjustment	4.3	4.6
Net pension cost (income)	9.5	10.5
Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period	2.3	2.8
Interest cost on projected benefit obligation	2.2	2.1
Expected return on plan assets	(3.5)	(3.4)
Amortization of actuarial gain (loss)	0.2	0.6
Subtotal	1.2	2.1
Regulatory adjustment	0.1	(0.8)
Net pension cost (income)	1.3	1.3
Spire Missouri | Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period	3.3	3.3
Interest cost on projected benefit obligation	4.9	4.8
Expected return on plan assets	(7.2)	(7.3)
Amortization of prior service credit	0.2	0.2
Amortization of actuarial gain (loss)	2.6	2.9
Subtotal	3.8	3.9
Regulatory adjustment	2.4	2.8
Net pension cost (income)	6.2	6.7
Spire Missouri | Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period	2.2	2.6
Interest cost on projected benefit obligation	1.8	1.7
Expected return on plan assets	(2.4)	(2.3)
Amortization of prior service credit	0.1	0.1
Amortization of actuarial gain (loss)	0.2	0.6
Subtotal	1.9	2.7
Regulatory adjustment	0.5	(0.4)
Net pension cost (income)	2.4	2.3
Spire Alabama | Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period	1.6	1.6
Interest cost on projected benefit obligation	1.4	1.5
Expected return on plan assets	(1.7)	(1.8)
Amortization of prior service credit	(0.5)	0.0
Amortization of actuarial gain (loss)	0.5	0.5
Subtotal	1.3	1.8
Regulatory adjustment	1.7	1.6
Net pension cost (income)	3.0	3.4
Spire Alabama | Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period	0.0	0.1
Interest cost on projected benefit obligation	0.4	0.4
Expected return on plan assets	(1.0)	(1.1)
Amortization of prior service credit	(0.1)	(0.1)
Subtotal	(0.7)	(0.7)
Regulatory adjustment	(0.4)	(0.4)
Net pension cost (income)	$ (1.1)	$ (1.1)