Note 13 - Pension Plans and Other Postretirement Benefits - Reconciliation of Pension Benefit Obligation (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Settlement benefits paid	$ 0	$ 0
Pension Plan [Member]
Benefit obligation, beginning of year	464,400	689,600
Service cost – benefits earned during the period	16,300	20,000	$ 21,700
Interest cost on projected benefit obligation	25,100	21,500	20,700
Actuarial (gain) loss	(1,100)	(173,900)
Settlement loss	9,700	33,400
Settlement benefits paid	(48,800)	(109,300)
Regular benefits paid	(26,900)	(16,900)
Benefit obligation, end of year	438,700	464,400	689,600
Accumulated benefit obligation, end of year	433,100	457,100
Pension Plan [Member] | Spire Missouri [Member]
Benefit obligation, beginning of year	320,600	479,000
Service cost – benefits earned during the period	11,400	14,400	15,400
Interest cost on projected benefit obligation	17,800	14,700	14,200
Actuarial (gain) loss	(4,900)	(116,200)
Settlement loss	600	27,900
Settlement benefits paid	(9,900)	(87,000)
Regular benefits paid	(22,400)	(12,200)
Benefit obligation, end of year	313,200	320,600	479,000
Accumulated benefit obligation, end of year	309,500	315,000
Pension Plan [Member] | Spire Alabama Inc [Member]
Benefit obligation, beginning of year	97,800	149,400
Service cost – benefits earned during the period	4,200	4,800	5,500
Interest cost on projected benefit obligation	4,800	4,700	4,600
Actuarial (gain) loss	5,400	(42,400)
Settlement loss	9,100	5,600
Settlement benefits paid	(38,800)	(22,300)
Regular benefits paid	(1,900)	(2,000)
Benefit obligation, end of year	80,600	97,800	149,400
Accumulated benefit obligation, end of year	78,800	96,200
Other Postretirement Benefits Plan [Member]
Benefit obligation, beginning of year	148,100	204,200
Service cost – benefits earned during the period	4,800	7,500	7,300
Interest cost on projected benefit obligation	8,500	6,000	6,000
Actuarial (gain) loss	(10,600)	(52,600)
Regular benefits paid	(8,400)	(11,000)
Benefit obligation, end of year	142,600	148,100	204,200
Plan amendments	0	(6,200)
Retiree drug subsidy program	200	200
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member]
Benefit obligation, beginning of year	114,800	151,700
Service cost – benefits earned during the period	4,000	6,300	6,200
Interest cost on projected benefit obligation	6,500	4,500	4,500
Actuarial (gain) loss	(13,400)	(37,700)
Regular benefits paid	(6,800)	(9,200)
Benefit obligation, end of year	105,300	114,800	151,700
Plan amendments	0	(1,000)
Retiree drug subsidy program	200	200
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member]
Benefit obligation, beginning of year	29,900	48,000
Service cost – benefits earned during the period	700	1,100	900
Interest cost on projected benefit obligation	1,800	1,400	1,300
Actuarial (gain) loss	4,200	(13,600)
Regular benefits paid	(1,500)	(1,800)
Benefit obligation, end of year	35,100	29,900	$ 48,000
Plan amendments	0	(5,200)
Retiree drug subsidy program	$ 0	$ 0