Pension Plans and Other Postretirement Benefits - Pre-tax Amounts Recognized in Accumulated Other Comprehensive Loss Not Yet Recognized as Components of Net Periodic Pension Costs (Details) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020
Pension Plans
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	$ 194.6	$ 235.6
Prior service (credit) cost	(40.3)	(25.6)
Subtotal	154.3	210.0
Adjustments for amounts included in regulatory assets	(149.5)	(206.7)
Total	4.8	3.3
Other Postretirement Plans
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	(101.2)	(61.7)
Prior service (credit) cost	14.6	15.5
Subtotal	(86.6)	(46.2)
Adjustments for amounts included in regulatory assets	86.6	46.2
Spire Missouri | Pension Plans
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	129.6	152.9
Prior service (credit) cost	(20.1)	(2.8)
Subtotal	109.5	150.1
Adjustments for amounts included in regulatory assets	(104.7)	(146.8)
Total	4.8	3.3
Spire Missouri | Other Postretirement Plans
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	(85.8)	(58.3)
Prior service (credit) cost	9.7	10.4
Subtotal	(76.1)	(47.9)
Adjustments for amounts included in regulatory assets	76.1	47.9
Spire Alabama | Pension Plans
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	66.1	78.1
Prior service (credit) cost	(18.7)	(21.1)
Subtotal	47.4	57.0
Adjustments for amounts included in regulatory assets	(47.4)	(57.0)
Total	0.0	0.0
Spire Alabama | Other Postretirement Plans
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	(11.5)	(1.5)
Prior service (credit) cost	4.9	5.1
Subtotal	(6.6)	3.6
Adjustments for amounts included in regulatory assets	$ 6.6	$ (3.6)