Pension Plans and Other Postretirement Benefits - Reconciliation of the Beginning and Ending Balances of Benefit Obligation (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Pension Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	$ 751.4	$ 664.6
Service cost	22.5	19.3	$ 20.2
Interest cost	22.6	28.2	27.4
Actuarial loss (gain)	37.5	104.8
Plan amendments	(4.4)	(10.2)
Settlement loss	16.5
Settlement benefits paid	(89.3)
Regular benefits paid	(24.2)	(55.3)
Benefit obligation, end of year	732.6	751.4	664.6
Accumulated benefit obligation, end of year	699.3	712.9
Pension Plans | Spire Missouri
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	538.4	473.3
Service cost	15.7	12.4	12.7
Interest cost	15.8	19.8	19.5
Actuarial loss (gain)	15.6	76.0
Plan amendments	(4.4)	(3.7)
Settlement loss	16.5
Settlement benefits paid	(74.5)
Regular benefits paid	(17.9)	(39.4)
Benefit obligation, end of year	505.2	538.4	473.3
Accumulated benefit obligation, end of year	473.7	500.2
Pension Plans | Spire Alabama
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	152.5	136.3
Service cost	6.1	6.2	6.4
Interest cost	4.9	6.0	5.5
Actuarial loss (gain)	18.4	23.7
Plan amendments		(6.5)
Settlement benefits paid	(14.8)
Regular benefits paid	(3.6)	(13.2)
Benefit obligation, end of year	163.5	152.5	136.3
Accumulated benefit obligation, end of year	161.8	152.3
Other Postretirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	197.3	208.1
Service cost	5.9	7.4	9.4
Interest cost	6.3	9.0	8.6
Actuarial loss (gain)	0.2	(18.9)
Plan amendments	15.8	5.5
Retiree drug subsidy program		0.2
Regular benefits paid	(13.2)	(14.0)
Benefit obligation, end of year	212.3	197.3	208.1
Other Postretirement Plans | Spire Missouri
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	147.9	158.8
Service cost	5.3	6.8	9.0
Interest cost	4.7	6.9	6.9
Actuarial loss (gain)	1.7	(19.5)
Plan amendments	9.5	4.9
Retiree drug subsidy program		0.2
Regular benefits paid	(10.7)	(10.2)
Benefit obligation, end of year	158.4	147.9	158.8
Other Postretirement Plans | Spire Alabama
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	43.4	43.1
Service cost	0.4	0.4	0.2
Interest cost	1.4	1.8	1.5
Actuarial loss (gain)	(0.7)	1.2
Plan amendments	6.3	0.6
Regular benefits paid	(2.5)	(3.7)
Benefit obligation, end of year	$ 48.3	$ 43.4	$ 43.1