Pension Plans and Other Postretirement Benefits - Schedule of Net Periodic Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	$ 5.5	$ 5.7	$ 16.4	$ 16.8
Interest cost on projected benefit obligation	5.5	5.5	15.3	17.2
Expected return on plan assets	(8.5)	(8.5)	(23.6)	(26.7)
Amortization of prior service cost (credit)	(0.9)	(0.6)	(2.4)	(1.8)
Amortization of actuarial (gain) loss	3.8	3.3	11.6	10.9
Loss on lump-sum settlements	11.2	2.3	15.0	23.3
Subtotal	16.6	7.7	32.3	39.7
Regulatory adjustment	1.3	7.4	19.9	5.7
Net pension cost	17.9	15.1	52.2	45.4
Other Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	1.7	1.5	5.4	4.4
Interest cost on projected benefit obligation	1.4	1.5	4.4	4.6
Expected return on plan assets	(4.0)	(4.1)	(12.1)	(12.3)
Amortization of prior service cost (credit)	0.3	(0.1)	0.8	(0.4)
Amortization of actuarial (gain) loss	(0.4)	(0.6)	(1.2)	(1.6)
Subtotal	(1.0)	(1.8)	(2.7)	(5.3)
Regulatory adjustment	3.5	4.0	10.1	12.0
Net pension cost	2.5	2.2	7.4	6.7
Spire Missouri | Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	3.9	4.0	11.7	11.7
Interest cost on projected benefit obligation	3.5	3.8	10.5	12.0
Expected return on plan assets	(5.7)	(5.9)	(16.9)	(18.8)
Amortization of prior service cost (credit)	(0.2)		(0.5)	0.1
Amortization of actuarial (gain) loss	2.8	2.6	8.5	8.6
Loss on lump-sum settlements	9.1	2.3	9.1	23.3
Subtotal	13.4	6.8	22.4	36.9
Regulatory adjustment	(1.4)	5.4	13.7	(0.1)
Net pension cost	12.0	12.2	36.1	36.8
Spire Missouri | Other Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	1.5	1.4	4.6	4.0
Interest cost on projected benefit obligation	1.1	1.2	3.3	3.5
Expected return on plan assets	(2.6)	(2.8)	(8.1)	(8.5)
Amortization of prior service cost (credit)	0.2	(0.1)	0.6	(0.2)
Amortization of actuarial (gain) loss	(0.4)	(0.6)	(1.1)	(1.6)
Subtotal	(0.2)	(0.9)	(0.7)	(2.8)
Regulatory adjustment	3.8	4.4	11.3	13.3
Net pension cost	3.6	3.5	10.6	10.5
Spire Alabama | Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	1.3	1.6	4.2	4.6
Interest cost on projected benefit obligation	1.1	1.2	3.5	3.7
Expected return on plan assets	(1.4)	(1.7)	(4.5)	(5.2)
Amortization of prior service cost (credit)	(0.6)	(0.6)	(1.8)	(1.8)
Amortization of actuarial (gain) loss	1.0	0.7	3.1	2.3
Loss on lump-sum settlements	2.1		5.9
Subtotal	3.5	1.2	10.4	3.6
Regulatory adjustment	2.5	1.7	5.6	5.1
Net pension cost	6.0	2.9	16.0	8.7
Spire Alabama | Other Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	0.2	0.1	0.7	0.3
Interest cost on projected benefit obligation	0.3	0.3	1.0	1.0
Expected return on plan assets	(1.3)	(1.2)	(3.8)	(3.6)
Amortization of prior service cost (credit)	0.1		0.2	(0.2)
Subtotal	(0.7)	(0.8)	(1.9)	(2.5)
Regulatory adjustment	(0.4)	(0.4)	(1.3)	(1.3)
Net pension cost	$ (1.1)	$ (1.2)	$ (3.2)	$ (3.8)