Pension Plans and Other Postretirement Benefits - Fair Value of Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Pension Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	$ 521.8	$ 499.2
Actual return on plan assets	21.1	50.1
Employer contributions	43.7	27.8
Settlement benefits paid	(89.3)
Regular benefits paid	(24.2)	(55.3)
Fair value of plan assets, end of year	473.1	521.8
Funded status of plans, end of year	(259.5)	(229.6)
Pension Plans | Spire Missouri
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	379.2	349.1
Actual return on plan assets	19.1	42.4
Employer contributions	30.3	27.1
Settlement benefits paid	(74.5)
Regular benefits paid	(17.9)	(39.4)
Fair value of plan assets, end of year	336.2	379.2
Funded status of plans, end of year	(169.0)	(159.2)
Pension Plans | Spire Alabama
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	93.3	101.3
Actual return on plan assets	1.0	4.7
Employer contributions	12.7	0.5
Settlement benefits paid	(14.8)
Regular benefits paid	(3.6)	(13.2)
Fair value of plan assets, end of year	88.6	93.3
Funded status of plans, end of year	(74.9)	(59.2)
Other Postretirement Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	279.8	283.5
Actual return on plan assets	24.2	9.7
Employer contributions	0.2	0.6
Regular benefits paid	(13.2)	(14.0)
Fair value of plan assets, end of year	291.0	279.8
Funded status of plans, end of year	78.7	82.5
Other Postretirement Plans | Spire Missouri
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	188.9	189.5
Actual return on plan assets	20.8	9.0
Employer contributions	0.2	0.6
Regular benefits paid	(10.7)	(10.2)
Fair value of plan assets, end of year	199.2	188.9
Funded status of plans, end of year	40.8	41.0
Other Postretirement Plans | Spire Alabama
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	86.4	89.6
Actual return on plan assets	3.1	0.5
Regular benefits paid	(2.5)	(3.7)
Fair value of plan assets, end of year	87.0	86.4
Funded status of plans, end of year	$ 38.7	$ 43.0