PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Net Periodic Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	$ 20.2	$ 20.5	$ 15.3
Interest cost on projected benefit obligation	27.4	27.9	28.0
Expected return on plan assets	(37.0)	(38.5)	(34.9)
Amortization of prior service cost (credit)	(0.9)	1.0	0.4
Amortization of actuarial loss	10.9	12.5	8.0
Loss on lump-sum settlements and curtailments	18.6	17.9	3.3
Special termination benefits	0.0	0.9	1.6
Subtotal	39.2	42.2	21.7
Regulatory adjustment	37.4	(2.4)	17.8
Net benefit cost	76.6	39.8	39.5
Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	9.4	11.0	10.9
Interest cost on projected benefit obligation	8.6	8.6	10.2
Expected return on plan assets	(13.9)	(13.6)	(13.5)
Amortization of prior service cost (credit)	(0.1)	0.0	0.3
Amortization of actuarial loss	0.8	2.5	3.6
Special termination benefits	0.0	0.0	2.6
Subtotal	4.8	8.5	14.1
Regulatory adjustment	2.2	(3.2)	(6.6)
Net benefit cost	7.0	5.3	7.5
Spire Missouri | Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	12.7	12.7	10.0
Interest cost on projected benefit obligation	19.5	19.5	21.7
Expected return on plan assets	(27.2)	(28.1)	(26.7)
Amortization of prior service cost (credit)	0.9	1.0	0.4
Amortization of actuarial loss	9.4	10.7	7.9
Loss on lump-sum settlements and curtailments	16.1	13.5	0.0
Special termination benefits	0.0	0.0	1.6
Subtotal	31.4	29.3	14.9
Regulatory adjustment	32.1	(4.1)	11.7
Net benefit cost	63.5	25.2	26.6
Spire Missouri | Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	9.0	10.4	10.6
Interest cost on projected benefit obligation	6.9	6.8	8.1
Expected return on plan assets	(9.6)	(9.0)	(8.5)
Amortization of prior service cost (credit)	0.3	0.2	0.3
Amortization of actuarial loss	0.9	2.6	3.8
Special termination benefits	0.0	0.0	2.6
Subtotal	7.5	11.0	16.9
Regulatory adjustment	3.9	(1.5)	(4.8)
Net benefit cost	11.4	9.5	12.1
Spire Alabama | Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	6.4	6.2	5.3
Interest cost on projected benefit obligation	5.5	6.1	6.3
Expected return on plan assets	(6.5)	(7.2)	(8.2)
Amortization of prior service cost (credit)	(1.8)	0.0	0.0
Amortization of actuarial loss	1.5	1.8	0.1
Loss on lump-sum settlements and curtailments	2.4	4.6	3.3
Special termination benefits	0.0	0.0	0.0
Subtotal	7.5	11.5	6.8
Regulatory adjustment	4.5	1.8	6.1
Net benefit cost	12.0	13.3	12.9
Spire Alabama | Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	0.2	0.3	0.3
Interest cost on projected benefit obligation	1.5	1.6	2.1
Expected return on plan assets	(4.1)	(4.4)	(5.0)
Amortization of prior service cost (credit)	(0.4)	(0.2)	0.0
Amortization of actuarial loss	(0.1)	(0.1)	(0.2)
Special termination benefits	0.0	0.0	0.0
Subtotal	(2.9)	(2.8)	(2.8)
Regulatory adjustment	(1.8)	(1.8)	(1.8)
Net benefit cost	$ (4.7)	$ (4.6)	$ (4.6)