Pension Plans and Other Postretirement Benefits - Schedule of Changes in Plan Assets and Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Amortization of actuarial gain (loss)	$ (0.9)	$ 0.4	$ 0.4
Pension Plans
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial loss (gain)	90.9	(1.4)	14.1
Amortization of actuarial gain (loss)	(9.3)	(10.9)	(12.5)
Acceleration of loss recognized due to settlement	0.0	(18.5)	(18.2)
Current year service credit	(10.2)	(0.1)	0.0
Current year prior service credit (cost)	0.0	0.0	(20.7)
Transfer due to merger	0.0	0.1	0.0
Amortization of prior service credit (cost)	1.1	(2.9)	(1.0)
Amortization of transition asset	0.0	1.8	0.0
Subtotal	72.5	(31.9)	(38.3)
Regulatory adjustment	(71.7)	31.6	38.0
Total recognized in OCI	0.8	(0.3)	(0.3)
Pension Plans | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial loss (gain)	59.0	2.2	14.8
Amortization of actuarial gain (loss)	(8.7)	(9.4)	(10.7)
Acceleration of loss recognized due to settlement	0.0	(16.1)	(13.5)
Current year service credit	(3.7)	0.0	0.0
Current year prior service credit (cost)	0.0	0.0	0.0
Transfer due to merger	0.0	0.1	0.0
Amortization of prior service credit (cost)	(0.9)	(0.9)	(1.0)
Amortization of transition asset	0.0	0.0	0.0
Subtotal	45.7	(24.1)	(10.4)
Regulatory adjustment	(44.9)	23.8	10.1
Total recognized in OCI	0.8	(0.3)	(0.3)
Pension Plans | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial loss (gain)	26.3	(0.6)	3.3
Amortization of actuarial gain (loss)	(0.8)	(1.5)	(1.8)
Acceleration of loss recognized due to settlement	0.0	(2.4)	(4.5)
Current year service credit	(6.5)	0.0	0.0
Current year prior service credit (cost)	0.0	0.0	(20.7)
Transfer due to merger	0.0	0.0	0.0
Amortization of prior service credit (cost)	1.8	0.0	0.0
Amortization of transition asset	0.0	1.8	0.0
Subtotal	20.8	(2.7)	(23.7)
Regulatory adjustment	(20.8)	2.7	23.7
Total recognized in OCI	0.0	0.0	0.0
Other Postretirement Plans
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial loss (gain)	(12.2)	(45.1)	(34.1)
Amortization of actuarial gain (loss)	0.5	(0.8)	(2.5)
Current year prior service credit (cost)	5.5	0.0	(1.4)
Amortization of current year service cost	0.4	0.0	0.0
Amortization of prior service credit (cost)	(0.3)	0.1	0.0
Subtotal	(6.1)	(45.8)	(38.0)
Regulatory adjustment	6.1	45.8	38.0
Total recognized in OCI	0.0	0.0	0.0
Other Postretirement Plans | Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial loss (gain)	(17.3)	(47.1)	(28.5)
Amortization of actuarial gain (loss)	0.5	(0.9)	(2.6)
Current year prior service credit (cost)	4.9	0.0	0.0
Amortization of current year service cost	0.0	0.0	0.0
Amortization of prior service credit (cost)	(0.3)	(0.3)	(0.2)
Subtotal	(12.2)	(48.3)	(31.3)
Regulatory adjustment	12.2	48.3	31.3
Total recognized in OCI	0.0	0.0	0.0
Other Postretirement Plans | Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial loss (gain)	5.5	1.6	(4.5)
Amortization of actuarial gain (loss)	0.0	0.1	0.1
Current year prior service credit (cost)	0.6	0.0	(1.4)
Amortization of current year service cost	0.4	0.0	0.0
Amortization of prior service credit (cost)	0.0	0.4	0.2
Subtotal	6.5	2.1	(5.6)
Regulatory adjustment	(6.5)	(2.1)	5.6
Total recognized in OCI	$ 0.0	$ 0.0	$ 0.0