Pension Plans and Other Postretirement Benefits - Schedule of Net Periodic Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	$ 4.8	$ 5.2
Interest cost on projected benefit obligation	7.1	6.9
Expected return on plan assets	(9.1)	(9.7)
Amortization of prior service cost (credit)	(0.3)	(0.3)
Amortization of actuarial (gain) loss	2.3	3.1
Subtotal	4.8	5.2
Regulatory adjustment	9.9	4.3
Net pension cost	14.7	9.5
Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	1.9	2.3
Interest cost on projected benefit obligation	2.3	2.2
Expected return on plan assets	(4.1)	(3.5)
Amortization of actuarial (gain) loss	(0.1)	0.2
Subtotal		1.2
Regulatory adjustment	2.4	0.1
Net pension cost	2.4	1.3
Spire Missouri | Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	3.1	3.3
Interest cost on projected benefit obligation	5.0	4.9
Expected return on plan assets	(6.4)	(7.2)
Amortization of prior service cost (credit)	0.2	0.2
Amortization of actuarial (gain) loss	2.1	2.6
Subtotal	4.0	3.8
Regulatory adjustment	8.0	2.4
Net pension cost	12.0	6.2
Spire Missouri | Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	1.7	2.2
Interest cost on projected benefit obligation	1.7	1.8
Expected return on plan assets	(2.8)	(2.4)
Amortization of prior service cost (credit)	0.1	0.1
Subtotal	0.6	1.9
Regulatory adjustment	2.9	0.5
Net pension cost	3.5	2.4
Spire Alabama | Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	1.5	1.6
Interest cost on projected benefit obligation	1.5	1.4
Expected return on plan assets	(1.8)	(1.7)
Amortization of prior service cost (credit)	(0.4)	(0.5)
Amortization of actuarial (gain) loss	0.2	0.5
Subtotal	1.0	1.3
Regulatory adjustment	1.7	1.7
Net pension cost	2.7	3.0
Spire Alabama | Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	0.1
Interest cost on projected benefit obligation	0.5	0.4
Expected return on plan assets	(1.2)	(1.0)
Amortization of prior service cost (credit)	(0.1)	(0.1)
Subtotal	(0.7)	(0.7)
Regulatory adjustment	(0.5)	(0.4)
Net pension cost	$ (1.2)	$ (1.1)