Pension Plans and Other Postretirement Benefits - Reconciliation of the Beginning and Ending Balances of Benefit Obligation (Details) - USD ($)	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Settlement benefits paid	$ 0
Pension Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	732,600,000	$ 751,400,000
Service cost	21,700,000	22,500,000	$ 19,300,000
Interest cost	20,700,000	22,600,000	28,200,000
Actuarial loss (gain)	6,600,000	37,500,000
Plan amendments	(17,900,000)	(4,400,000)
Settlement loss	12,300,000	16,500,000
Settlement benefits paid	(67,600,000)	(89,300,000)
Regular benefits paid	(18,800,000)	(24,200,000)
Benefit obligation, end of year	689,600,000	732,600,000	751,400,000
Accumulated benefit obligation, end of year	673,300,000	699,300,000
Pension Plans | Spire Missouri
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	505,200,000	538,400,000
Service cost	15,400,000	15,700,000	12,400,000
Interest cost	14,200,000	15,800,000	19,800,000
Actuarial loss (gain)	11,000,000.0	15,600,000
Plan amendments	(17,900,000)	(4,400,000)
Settlement loss	8,200,000	16,500,000
Settlement benefits paid	(44,600,000)	(74,500,000)
Regular benefits paid	(12,500,000)	(17,900,000)
Benefit obligation, end of year	479,000,000.0	505,200,000	538,400,000
Accumulated benefit obligation, end of year	465,400,000	473,700,000
Pension Plans | Spire Alabama
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	163,500,000	152,500,000
Service cost	5,500,000	6,100,000	6,200,000
Interest cost	4,600,000	4,900,000	6,000,000.0
Actuarial loss (gain)	(1,800,000)	18,400,000
Settlement loss	4,100,000
Settlement benefits paid	(22,900,000)	(14,800,000)
Regular benefits paid	(3,600,000)	(3,600,000)
Benefit obligation, end of year	149,400,000	163,500,000	152,500,000
Accumulated benefit obligation, end of year	146,900,000	161,800,000
Other Postretirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	212,300,000	197,300,000
Service cost	7,300,000	5,900,000	7,400,000
Interest cost	6,000,000.0	6,300,000	9,000,000.0
Actuarial loss (gain)	(7,500,000)	200,000
Plan amendments		15,800,000
Regular benefits paid	(13,900,000)	(13,200,000)
Benefit obligation, end of year	204,200,000	212,300,000	197,300,000
Other Postretirement Plans | Spire Missouri
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	158,400,000	147,900,000
Service cost	6,200,000	5,300,000	6,800,000
Interest cost	4,500,000	4,700,000	6,900,000
Actuarial loss (gain)	(6,700,000)	1,700,000
Plan amendments		9,500,000
Regular benefits paid	(10,700,000)	(10,700,000)
Benefit obligation, end of year	151,700,000	158,400,000	147,900,000
Other Postretirement Plans | Spire Alabama
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	48,300,000	43,400,000
Service cost	900,000	400,000	400,000
Interest cost	1,300,000	1,400,000	1,800,000
Actuarial loss (gain)	700,000	(700,000)
Plan amendments		6,300,000
Regular benefits paid	(3,200,000)	(2,500,000)
Benefit obligation, end of year	$ 48,000,000.0	$ 48,300,000	$ 43,400,000