Pension Plans and Other Postretirement Benefits - Fair Value of Plan Assets (Details) - USD ($)	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Settlement benefits paid	$ 0
Pension Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	473,100,000	$ 521,800,000
Actual return on plan assets	58,800,000	21,100,000
Employer contributions	53,400,000	43,700,000
Settlement benefits paid	(67,600,000)	(89,300,000)
Regular benefits paid	(18,800,000)	(24,200,000)
Fair value of plan assets, end of year	498,900,000	473,100,000
Funded status of plans, end of year	(190,700,000)	(259,500,000)
Pension Plans | Spire Missouri
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	336,200,000	379,200,000
Actual return on plan assets	42,700,000	19,100,000
Employer contributions	42,200,000	30,300,000
Settlement benefits paid	(44,600,000)	(74,500,000)
Regular benefits paid	(12,500,000)	(17,900,000)
Fair value of plan assets, end of year	364,000,000.0	336,200,000
Funded status of plans, end of year	(115,000,000.0)	(169,000,000.0)
Pension Plans | Spire Alabama
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	88,600,000	93,300,000
Actual return on plan assets	9,600,000	1,000,000.0
Employer contributions	11,100,000	12,700,000
Settlement benefits paid	(22,900,000)	(14,800,000)
Regular benefits paid	(3,600,000)	(3,600,000)
Fair value of plan assets, end of year	82,800,000	88,600,000
Funded status of plans, end of year	(66,600,000)	(74,900,000)
Other Postretirement Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	291,000,000.0	279,800,000
Actual return on plan assets	49,700,000	24,200,000
Employer contributions	100,000	200,000
Regular benefits paid	(13,900,000)	(13,200,000)
Fair value of plan assets, end of year	326,900,000	291,000,000.0
Funded status of plans, end of year	122,700,000	78,700,000
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	199,200,000	188,900,000
Actual return on plan assets	33,100,000	20,800,000
Employer contributions	100,000	200,000
Regular benefits paid	(10,700,000)	(10,700,000)
Fair value of plan assets, end of year	221,700,000	199,200,000
Funded status of plans, end of year	70,000,000.0	40,800,000
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	87,000,000.0	86,400,000
Actual return on plan assets	15,600,000	3,100,000
Regular benefits paid	(3,200,000)	(2,500,000)
Fair value of plan assets, end of year	99,400,000	87,000,000.0
Funded status of plans, end of year	$ 51,400,000	$ 38,700,000