Pension Plans and Other Postretirement Benefits - Schedule of Changes in Plan Assets and Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Amortization of actuarial gain (loss)	$ (0.5)	$ (0.9)	$ 0.4
Pension Plans
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial loss (gain)	68.0	90.9	(1.4)
Amortization of actuarial gain (loss)	(14.4)	(9.3)	(10.9)
Acceleration of loss recognized due to settlement	(31.7)	0.0	(18.5)
Current year service credit	(4.4)	(10.2)	(0.1)
Transfer due to merger	0.0	0.0	0.1
Amortization of prior service credit (cost)	2.5	1.1	(2.9)
Amortization of transition asset	0.0	0.0	1.8
Subtotal	20.0	72.5	(31.9)
Regulatory adjustment	(19.5)	(71.7)	31.6
Total recognized in OCI	0.5	0.8	(0.3)
Other Postretirement Plans
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial loss (gain)	(7.3)	(12.2)	(45.1)
Amortization of actuarial gain (loss)	2.0	0.5	(0.8)
Current year prior service credit (cost)	15.8	5.5	0.0
Amortization of current year service cost	0.0	0.4	0.0
Amortization of prior service credit (cost)	0.5	(0.3)	0.1
Subtotal	11.0	(6.1)	(45.8)
Regulatory adjustment	(11.0)	6.1	45.8
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 loss (gain)	37.8	59.0	2.2
Amortization of actuarial gain (loss)	(11.3)	(8.7)	(9.4)
Acceleration of loss recognized due to settlement	(26.6)	0.0	(16.1)
Current year service credit	(4.4)	(3.7)	0.0
Transfer due to merger	0.0	0.0	0.1
Amortization of prior service credit (cost)	(0.1)	(0.9)	(0.9)
Amortization of transition asset	0.0	0.0	0.0
Subtotal	(4.6)	45.7	(24.1)
Regulatory adjustment	5.1	(44.9)	23.8
Total recognized in OCI	0.5	0.8	(0.3)
Spire Missouri | Other Postretirement Plans
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial loss (gain)	(7.6)	(17.3)	(47.1)
Amortization of actuarial gain (loss)	2.0	0.5	(0.9)
Current year prior service credit (cost)	9.5	4.9	0.0
Amortization of current year service cost	0.0	0.0	0.0
Amortization of prior service credit (cost)	0.2	(0.3)	(0.3)
Subtotal	4.1	(12.2)	(48.3)
Regulatory adjustment	(4.1)	12.2	48.3
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 loss (gain)	24.4	26.3	(0.6)
Amortization of actuarial gain (loss)	(3.1)	(0.8)	(1.5)
Acceleration of loss recognized due to settlement	(5.1)	0.0	(2.4)
Current year service credit	0.0	(6.5)	0.0
Transfer due to merger	0.0	0.0	0.0
Amortization of prior service credit (cost)	2.4	1.8	0.0
Amortization of transition asset	0.0	0.0	1.8
Subtotal	18.6	20.8	(2.7)
Regulatory adjustment	(18.6)	(20.8)	2.7
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 loss (gain)	1.1	5.5	1.6
Amortization of actuarial gain (loss)	0.0	0.0	0.1
Current year prior service credit (cost)	6.3	0.6	0.0
Amortization of current year service cost	0.0	0.4	0.0
Amortization of prior service credit (cost)	0.3	0.0	0.4
Subtotal	7.7	6.5	2.1
Regulatory adjustment	(7.7)	(6.5)	(2.1)
Total recognized in OCI	$ 0.0	$ 0.0	$ 0.0