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, 2019	Sep. 30, 2018	Sep. 30, 2017
Pension Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	$ 664.6	$ 748.8
Service cost	19.3	20.2	$ 20.5
Interest cost	28.2	27.4	27.9
Actuarial loss (gain)	104.8	(34.9)
Plan amendments	(10.2)	(2.0)
Settlement loss		7.6
Settlement benefits paid		(82.8)	(62.2)
Regular benefits paid	(55.3)	(19.7)
Benefit obligation, end of year	751.4	664.6	748.8
Accumulated benefit obligation, end of year	712.9	636.0
Pension Plans | Spire Missouri
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	473.3	539.6
Service cost	12.4	12.7	12.7
Interest cost	19.8	19.5	19.5
Actuarial loss (gain)	76.0	(26.0)
Plan amendments	(3.7)
Plan mergers		(0.3)
Settlement loss		7.6
Settlement benefits paid		(65.5)	(43.5)
Regular benefits paid	(39.4)	(14.3)
Benefit obligation, end of year	538.4	473.3	539.6
Accumulated benefit obligation, end of year	500.2	446.5
Pension Plans | Spire Alabama
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	136.3	148.2
Service cost	6.2	6.4	6.2
Interest cost	6.0	5.5	6.1
Actuarial loss (gain)	23.7	(3.8)
Plan amendments	(6.5)
Settlement benefits paid		(17.3)	(18.7)
Regular benefits paid	(13.2)	(2.7)
Benefit obligation, end of year	152.5	136.3	148.2
Accumulated benefit obligation, end of year	152.3	134.5
Other Postretirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	208.1	238.5
Service cost	7.4	9.4	11.0
Interest cost	9.0	8.6	8.6
Actuarial loss (gain)	(18.9)	(37.5)
Plan amendments	5.5
Retiree drug subsidy program	0.2	0.2
Regular benefits paid	(14.0)	(11.1)
Benefit obligation, end of year	197.3	208.1	238.5
Other Postretirement Plans | Spire Missouri
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	158.8	192.5
Service cost	6.8	9.0	10.4
Interest cost	6.9	6.9	6.8
Actuarial loss (gain)	(19.5)	(41.1)
Plan amendments	4.9
Retiree drug subsidy program	0.2	0.1
Regular benefits paid	(10.2)	(8.6)
Benefit obligation, end of year	147.9	158.8	192.5
Other Postretirement Plans | Spire Alabama
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	43.1	40.6
Service cost	0.4	0.2	0.3
Interest cost	1.8	1.5	1.6
Actuarial loss (gain)	1.2	3.1
Plan amendments	0.6
Retiree drug subsidy program		0.1
Regular benefits paid	(3.7)	(2.4)
Benefit obligation, end of year	$ 43.4	$ 43.1	$ 40.6