Pension Plans and Other Postretirement Benefits - Schedule of Net Periodic Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	$ 19.3	$ 20.2	$ 20.5
Interest cost on projected benefit obligation	28.2	27.4	27.9
Expected return on plan assets	(36.3)	(37.0)	(38.5)
Amortization of prior service (credit) cost	(1.1)	(0.9)	1.0
Amortization of actuarial (gain) loss	9.3	10.9	12.5
Loss on lump-sum settlements and curtailments	0.0	18.6	17.9
Special termination benefits	0.0	0.0	0.9
Subtotal	19.4	39.2	42.2
Regulatory adjustment	39.6	37.4	(2.4)
Net pension cost	59.0	76.6	39.8
Other Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	7.4	9.4	11.0
Interest cost on projected benefit obligation	9.0	8.6	8.6
Expected return on plan assets	(16.2)	(13.9)	(13.6)
Amortization of prior service (credit) cost	(0.1)	(0.1)	0.0
Amortization of actuarial (gain) loss	(0.5)	0.8	2.5
Subtotal	(0.4)	4.8	8.5
Regulatory adjustment	10.0	2.2	(3.2)
Net pension cost	9.6	7.0	5.3
Spire Missouri | Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	12.4	12.7	12.7
Interest cost on projected benefit obligation	19.8	19.5	19.5
Expected return on plan assets	(25.5)	(27.2)	(28.1)
Amortization of prior service (credit) cost	0.9	0.9	1.0
Amortization of actuarial (gain) loss	8.7	9.4	10.7
Loss on lump-sum settlements and curtailments	0.0	16.1	13.5
Special termination benefits	0.0	0.0	0.0
Subtotal	16.3	31.4	29.3
Regulatory adjustment	31.8	32.1	(4.1)
Net pension cost	48.1	63.5	25.2
Spire Missouri | Other Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	6.8	9.0	10.4
Interest cost on projected benefit obligation	6.9	6.9	6.8
Expected return on plan assets	(11.1)	(9.6)	(9.0)
Amortization of prior service (credit) cost	0.3	0.3	0.2
Amortization of actuarial (gain) loss	(0.5)	0.9	2.6
Subtotal	2.4	7.5	11.0
Regulatory adjustment	11.7	3.9	(1.5)
Net pension cost	14.1	11.4	9.5
Spire Alabama | Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	6.2	6.4	6.2
Interest cost on projected benefit obligation	6.0	5.5	6.1
Expected return on plan assets	(7.3)	(6.5)	(7.2)
Amortization of prior service (credit) cost	(1.8)	(1.8)	0.0
Amortization of actuarial (gain) loss	0.8	1.5	1.8
Loss on lump-sum settlements and curtailments	0.0	2.4	4.6
Special termination benefits	0.0	0.0	0.0
Subtotal	3.9	7.5	11.5
Regulatory adjustment	6.9	4.5	1.8
Net pension cost	10.8	12.0	13.3
Spire Alabama | Other Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	0.4	0.2	0.3
Interest cost on projected benefit obligation	1.8	1.5	1.6
Expected return on plan assets	(4.8)	(4.1)	(4.4)
Amortization of prior service (credit) cost	(0.4)	(0.4)	(0.2)
Amortization of actuarial (gain) loss	0.0	(0.1)	(0.1)
Subtotal	(3.0)	(2.9)	(2.8)
Regulatory adjustment	(1.8)	(1.8)	(1.8)
Net pension cost	$ (4.8)	$ (4.7)	$ (4.6)