Pension Plans and Other Postretirement Benefits - Schedule of Net Periodic Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	$ 4.8	$ 4.8	$ 14.5	$ 15.2
Interest cost on projected benefit obligation	7.1	6.5	21.2	20.3
Expected return on plan assets	(9.2)	(8.6)	(27.2)	(27.8)
Amortization of prior service cost (credit)	(0.3)	(0.2)	(0.9)	(0.7)
Amortization of actuarial (gain) loss	2.3	2.4	6.9	8.4
Loss on lump-sum settlements*		7.5		16.9
Subtotal	4.7	12.4	14.5	32.3
Regulatory adjustment	10.1	2.1	29.8	30.4
Net pension cost	14.8	14.5	44.3	62.7
Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	1.8	2.3	5.5	7.0
Interest cost on projected benefit obligation	2.3	2.2	6.7	6.6
Expected return on plan assets	(4.1)	(3.5)	(12.1)	(10.5)
Amortization of prior service cost (credit)			(0.1)	(0.1)
Amortization of actuarial (gain) loss	(0.1)	0.2	(0.4)	0.6
Subtotal	(0.1)	1.2	(0.4)	3.6
Regulatory adjustment	2.5	0.7	7.5	0.8
Net pension cost	2.4	1.9	7.1	4.4
Spire Missouri | Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	3.2	2.9	9.4	9.4
Interest cost on projected benefit obligation	5.0	4.6	14.9	14.4
Expected return on plan assets	(6.5)	(6.1)	(19.2)	(20.3)
Amortization of prior service cost (credit)	0.2	0.3	0.6	0.7
Amortization of actuarial (gain) loss	2.2	2.0	6.5	7.1
Loss on lump-sum settlements*		5.2		14.6
Subtotal	4.1	8.9	12.2	25.9
Regulatory adjustment	8.0	2.4	23.9	26.7
Net pension cost	12.1	11.3	36.1	52.6
Spire Missouri | Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	1.7	2.2	5.1	6.7
Interest cost on projected benefit obligation	1.8	1.7	5.2	5.3
Expected return on plan assets	(2.8)	(2.4)	(8.3)	(7.3)
Amortization of prior service cost (credit)	0.1	0.1	0.2	0.2
Amortization of actuarial (gain) loss	(0.1)	0.3	(0.4)	0.7
Subtotal	0.7	1.9	1.8	5.6
Regulatory adjustment	2.9	1.1	8.8	2.1
Net pension cost	3.6	3.0	10.6	7.7
Spire Alabama | Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	1.5	1.6	4.6	4.9
Interest cost on projected benefit obligation	1.5	1.3	4.5	4.1
Expected return on plan assets	(1.8)	(1.6)	(5.4)	(5.0)
Amortization of prior service cost (credit)	(0.5)	(0.5)	(1.4)	(1.4)
Amortization of actuarial (gain) loss	0.2	0.4	0.6	1.3
Loss on lump-sum settlements*		2.3		2.3
Subtotal	0.9	3.5	2.9	6.2
Regulatory adjustment	1.8	(0.6)	5.2	3.0
Net pension cost	2.7	2.9	8.1	9.2
Spire Alabama | Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	0.1	0.1	0.3	0.2
Interest cost on projected benefit obligation	0.4	0.4	1.3	1.1
Expected return on plan assets	(1.2)	(1.1)	(3.6)	(3.1)
Amortization of prior service cost (credit)	(0.1)	(0.1)	(0.3)	(0.3)
Amortization of actuarial (gain) loss		(0.1)		(0.1)
Subtotal	(0.8)	(0.8)	(2.3)	(2.2)
Regulatory adjustment	(0.4)	(0.5)	(1.3)	(1.4)
Net pension cost	$ (1.2)	$ (1.3)	$ (3.6)	$ (3.6)