PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Net Periodic Cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period	$ 5.2	$ 5.2	$ 10.4	$ 10.5
Interest cost on projected benefit obligation	6.9	6.9	13.8	13.8
Expected return on plan assets	(9.5)	(9.7)	(19.2)	(19.6)
Amortization of prior service credit	(0.2)	0.3	(0.5)	0.5
Amortization of actuarial gain (loss)	2.9	3.2	6.0	6.6
Loss on lump-sum settlements	9.4	11.9	9.4	11.9
Subtotal	14.7	17.8	19.9	23.7
Regulatory adjustment	24.0	(7.4)	28.3	(2.8)
Net pension cost (income)	38.7	10.4	48.2	20.9
Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period	2.4	2.7	4.7	5.5
Interest cost on projected benefit obligation	2.2	2.2	4.4	4.3
Expected return on plan assets	(3.5)	(3.4)	(7.0)	(6.8)
Amortization of prior service credit	(0.1)	0.0	(0.1)	0.0
Amortization of actuarial gain (loss)	0.2	0.6	0.4	1.2
Subtotal	1.2	2.1	2.4	4.2
Regulatory adjustment	0.0	(0.8)	0.1	(1.6)
Net pension cost (income)	1.2	1.3	2.5	2.6
Spire Missouri | Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period	3.2	3.3	6.5	6.6
Interest cost on projected benefit obligation	4.9	4.9	9.8	9.7
Expected return on plan assets	(7.0)	(7.1)	(14.2)	(14.4)
Amortization of prior service credit	0.2	0.3	0.4	0.5
Amortization of actuarial gain (loss)	2.5	2.8	5.1	5.7
Loss on lump-sum settlements	9.4	11.5	9.4	11.5
Subtotal	13.2	15.7	17.0	19.6
Regulatory adjustment	21.9	(8.6)	24.3	(5.8)
Net pension cost (income)	35.1	7.1	41.3	13.8
Spire Missouri | Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period	2.3	2.6	4.5	5.2
Interest cost on projected benefit obligation	1.8	1.7	3.6	3.4
Expected return on plan assets	(2.5)	(2.2)	(4.9)	(4.5)
Amortization of prior service credit	0.0	0.0	0.1	0.1
Amortization of actuarial gain (loss)	0.2	0.7	0.4	1.3
Subtotal	1.8	2.8	3.7	5.5
Regulatory adjustment	0.5	(0.3)	1.0	(0.7)
Net pension cost (income)	2.3	2.5	4.7	4.8
Spire Alabama | Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period	1.7	1.5	3.3	3.1
Interest cost on projected benefit obligation	1.4	1.5	2.8	3.0
Expected return on plan assets	(1.7)	(1.8)	(3.4)	(3.6)
Amortization of prior service credit	(0.4)	0.0	(0.9)	0.0
Amortization of actuarial gain (loss)	0.4	0.4	0.9	0.9
Loss on lump-sum settlements	0.0	0.4	0.0	0.4
Subtotal	1.4	2.0	2.7	3.8
Regulatory adjustment	1.9	1.1	3.6	2.7
Net pension cost (income)	3.3	3.1	6.3	6.5
Spire Alabama | Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period	0.1	0.1	0.1	0.2
Interest cost on projected benefit obligation	0.3	0.4	0.7	0.8
Expected return on plan assets	(1.0)	(1.1)	(2.0)	(2.2)
Amortization of prior service credit	(0.1)	0.0	(0.2)	(0.1)
Amortization of actuarial gain (loss)	0.0	(0.1)	0.0	(0.1)
Subtotal	(0.7)	(0.7)	(1.4)	(1.4)
Regulatory adjustment	(0.5)	(0.5)	(0.9)	(0.9)
Net pension cost (income)	$ (1.2)	$ (1.2)	$ (2.3)	$ (2.3)