Pension Plans and Other Postretirement Benefits - Schedule of Changes in Plan Assets and Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Pension Plans
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial (gain) loss	$ (8.1)	$ 68.0	$ 90.9
Amortization of actuarial gain (loss)	(14.9)	(14.4)	(9.3)
Acceleration of loss recognized due to settlement	(18.2)	(31.7)	0.0
Current year service credit	(17.9)	(4.4)	(10.2)
Amortization of prior service credit (cost)	3.1	2.5	1.1
Subtotal	(56.0)	20.0	72.5
Regulatory adjustment	57.3	(19.5)	(71.7)
Total recognized in OCI	1.3	0.5	0.8
Other Postretirement Plans
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial (gain) loss	(41.0)	(7.3)	(12.2)
Amortization of actuarial gain (loss)	1.6	2.0	0.5
Current year prior service credit (cost)	0.0	15.8	5.5
Amortization of current year service cost	0.0	0.0	0.4
Amortization of prior service credit (cost)	(1.0)	0.5	(0.3)
Subtotal	(40.4)	11.0	(6.1)
Regulatory adjustment	40.4	(11.0)	6.1
Total recognized in OCI	0.0	0.0	0.0
Spire Missouri | Pension Plans
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial (gain) loss	(0.9)	37.8	59.0
Amortization of actuarial gain (loss)	(11.0)	(11.3)	(8.7)
Acceleration of loss recognized due to settlement	(11.6)	(26.6)	0.0
Current year service credit	(17.9)	(4.4)	(3.7)
Amortization of prior service credit (cost)	0.6	(0.1)	(0.9)
Subtotal	(40.8)	(4.6)	45.7
Regulatory adjustment	42.1	5.1	(44.9)
Total recognized in OCI	1.3	0.5	0.8
Spire Missouri | Other Postretirement Plans
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial (gain) loss	(29.0)	(7.6)	(17.3)
Amortization of actuarial gain (loss)	1.5	2.0	0.5
Current year prior service credit (cost)	0.0	9.5	4.9
Amortization of current year service cost	0.0	0.0	0.0
Amortization of prior service credit (cost)	(0.7)	0.2	(0.3)
Subtotal	(28.2)	4.1	(12.2)
Regulatory adjustment	28.2	(4.1)	12.2
Total recognized in OCI	0.0	0.0	0.0
Spire Alabama | Pension Plans
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial (gain) loss	(1.5)	24.4	26.3
Amortization of actuarial gain (loss)	(3.9)	(3.1)	(0.8)
Acceleration of loss recognized due to settlement	(6.6)	(5.1)	0.0
Current year service credit	0.0	0.0	(6.5)
Amortization of prior service credit (cost)	2.3	2.4	1.8
Subtotal	(9.7)	18.6	20.8
Regulatory adjustment	9.7	(18.6)	(20.8)
Total recognized in OCI	0.0	0.0	0.0
Spire Alabama | Other Postretirement Plans
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial (gain) loss	(9.9)	1.1	5.5
Amortization of actuarial gain (loss)	0.0	0.0	0.0
Current year prior service credit (cost)	0.0	6.3	0.6
Amortization of current year service cost	0.0	0.0	0.4
Amortization of prior service credit (cost)	(0.3)	0.3	0.0
Subtotal	(10.2)	7.7	6.5
Regulatory adjustment	10.2	(7.7)	(6.5)
Total recognized in OCI	$ 0.0	$ 0.0	$ 0.0