Pension Plans and Other Postretirement Benefits - Schedule of Net Periodic Cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	$ 5.6	$ 4.9	$ 11.1	$ 9.7
Interest cost on projected benefit obligation	5.8	7.0	11.7	14.1
Expected return on plan assets	(9.0)	(8.9)	(18.2)	(18.0)
Amortization of prior service (credit) cost	(0.6)	(0.3)	(1.2)	(0.6)
Amortization of actuarial (gain) loss	3.9	2.3	7.6	4.6
Loss on lump-sum settlements	21.0		21.0
Subtotal	26.7	5.0	32.0	9.8
Regulatory adjustment	(11.3)	9.8	(1.7)	19.7
Net pension cost	15.4	14.8	30.3	29.5
Other Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	1.5	1.8	2.9	3.7
Interest cost on projected benefit obligation	1.5	2.1	3.1	4.4
Expected return on plan assets	(4.1)	(3.9)	(8.2)	(8.0)
Amortization of prior service (credit) cost	(0.1)	(0.1)	(0.3)	(0.1)
Amortization of actuarial (gain) loss	(0.5)	(0.2)	(1.0)	(0.3)
Subtotal	(1.7)	(0.3)	(3.5)	(0.3)
Regulatory adjustment	4.0	2.6	8.0	5.0
Net pension cost	2.3	2.3	4.5	4.7
Spire Missouri | Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	3.9	3.1	7.7	6.2
Interest cost on projected benefit obligation	4.0	4.9	8.2	9.9
Expected return on plan assets	(6.3)	(6.3)	(12.9)	(12.7)
Amortization of prior service (credit) cost	0.1	0.2	0.1	0.4
Amortization of actuarial (gain) loss	3.1	2.2	6.0	4.3
Loss on lump-sum settlements	21.0		21.0
Subtotal	25.8	4.1	30.1	8.1
Regulatory adjustment	(13.2)	7.9	(5.5)	15.9
Net pension cost	12.6	12.0	24.6	24.0
Spire Missouri | Other Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	1.3	1.7	2.6	3.4
Interest cost on projected benefit obligation	1.1	1.7	2.3	3.4
Expected return on plan assets	(2.9)	(2.7)	(5.7)	(5.5)
Amortization of prior service (credit) cost			(0.1)	0.1
Amortization of actuarial (gain) loss	(0.5)	(0.2)	(1.0)	(0.3)
Subtotal	(1.0)	0.5	(1.9)	1.1
Regulatory adjustment	4.5	3.0	8.9	5.9
Net pension cost	3.5	3.5	7.0	7.0
Spire Alabama | Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	1.5	1.6	3.0	3.1
Interest cost on projected benefit obligation	1.3	1.5	2.5	3.0
Expected return on plan assets	(1.8)	(1.8)	(3.5)	(3.6)
Amortization of prior service (credit) cost	(0.6)	(0.5)	(1.2)	(0.9)
Amortization of actuarial (gain) loss	0.8	0.2	1.6	0.4
Subtotal	1.2	1.0	2.4	2.0
Regulatory adjustment	1.7	1.7	3.4	3.4
Net pension cost	2.9	2.7	5.8	5.4
Spire Alabama | Other Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	0.1	0.1	0.2	0.2
Interest cost on projected benefit obligation	0.4	0.4	0.7	0.9
Expected return on plan assets	(1.2)	(1.2)	(2.4)	(2.4)
Amortization of prior service (credit) cost	(0.1)	(0.1)	(0.2)	(0.2)
Subtotal	(0.8)	(0.8)	(1.7)	(1.5)
Regulatory adjustment	(0.5)	(0.4)	(0.9)	(0.9)
Net pension cost	$ (1.3)	$ (1.2)	$ (2.6)	$ (2.4)