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, 2020	Sep. 30, 2019
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	$ 235.6	$ 213.7
Prior service (credit) cost	(25.6)	(23.7)
Subtotal	210.0	190.0
Adjustments for amounts included in regulatory assets	(206.7)	(187.1)
Total	3.3	2.9
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	(61.7)	(56.3)
Prior service (credit) cost	15.5	(1.0)
Subtotal	(46.2)	(57.3)
Adjustments for amounts included in regulatory assets	46.2	57.3
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	152.9	153.1
Prior service (credit) cost	(2.8)	1.7
Subtotal	150.1	154.8
Adjustments for amounts included in regulatory assets	(146.8)	(151.9)
Total	3.3	2.9
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	(58.3)	(52.6)
Prior service (credit) cost	10.4	0.6
Subtotal	(47.9)	(52.0)
Adjustments for amounts included in regulatory assets	47.9	52.0
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	78.1	61.9
Prior service (credit) cost	(21.1)	(23.5)
Subtotal	57.0	38.4
Adjustments for amounts included in regulatory assets	(57.0)	(38.4)
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	(1.5)	(2.6)
Prior service (credit) cost	5.1	(1.6)
Subtotal	3.6	(4.2)
Adjustments for amounts included in regulatory assets	$ (3.6)	$ 4.2