PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Net Periodic Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	$ 20.5	$ 15.3	$ 17.3
Interest cost on projected benefit obligation	27.9	28.0	29.5
Expected return on plan assets	(38.5)	(34.9)	(37.4)
Amortization of prior service cost	1.0	0.4	0.5
Amortization of actuarial loss	12.5	8.0	7.5
Loss on lump-sum settlements and curtailments	17.9	3.3	19.6
Special termination benefits	0.9	1.6	0.0
Subtotal	42.2	21.7	37.0
Regulatory adjustment	(2.4)	17.8	(2.1)
Net benefit cost	39.8	39.5	34.9
Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	11.0	10.9	12.8
Interest cost on projected benefit obligation	8.6	10.2	11.2
Expected return on plan assets	(13.6)	(13.5)	(13.2)
Amortization of prior service cost	0.0	0.3	0.8
Amortization of actuarial loss	2.5	3.6	5.1
Special termination benefits	0.0	2.6	0.0
Subtotal	8.5	14.1	16.7
Regulatory adjustment	(3.2)	(6.6)	(11.0)
Net benefit cost	5.3	7.5	5.7
Spire Missouri | Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	12.7	10.0	11.5
Interest cost on projected benefit obligation	19.5	21.7	23.3
Expected return on plan assets	(28.1)	(26.7)	(29.2)
Amortization of prior service cost	1.0	0.4	0.5
Amortization of actuarial loss	10.7	7.9	7.5
Loss on lump-sum settlements and curtailments	13.5	0.0	18.0
Special termination benefits	0.0	1.6	0.0
Subtotal	29.3	14.9	31.6
Regulatory adjustment	(4.1)	11.7	(5.2)
Net benefit cost	25.2	26.6	26.4
Spire Missouri | Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	10.4	10.6	12.3
Interest cost on projected benefit obligation	6.8	8.1	8.6
Expected return on plan assets	(9.0)	(8.5)	(8.1)
Amortization of prior service cost	0.2	0.3	0.8
Amortization of actuarial loss	2.6	3.8	5.1
Special termination benefits	0.0	2.6	0.0
Subtotal	11.0	16.9	18.7
Regulatory adjustment	(1.5)	(4.8)	(9.2)
Net benefit cost	9.5	12.1	9.5
Spire Alabama | Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	6.2	5.3	5.8
Interest cost on projected benefit obligation	6.1	6.3	6.2
Expected return on plan assets	(7.2)	(8.2)	(8.2)
Amortization of prior service cost	0.0	0.0	0.0
Amortization of actuarial loss	1.8	0.1	0.0
Loss on lump-sum settlements and curtailments	4.6	3.3	1.6
Special termination benefits	0.0	0.0	0.0
Subtotal	11.5	6.8	5.4
Regulatory adjustment	1.8	6.1	3.1
Net benefit cost	13.3	12.9	8.5
Spire Alabama | Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	0.3	0.3	0.5
Interest cost on projected benefit obligation	1.6	2.1	2.6
Expected return on plan assets	(4.4)	(5.0)	(5.1)
Amortization of prior service cost	(0.2)	0.0	0.0
Amortization of actuarial loss	(0.1)	(0.2)	0.0
Special termination benefits	0.0	0.0	0.0
Subtotal	(2.8)	(2.8)	(2.0)
Regulatory adjustment	(1.8)	(1.8)	(1.8)
Net benefit cost	$ (4.6)	$ (4.6)	$ (3.8)