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, 2019	Sep. 30, 2018
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	$ 213.7	$ 132.1
Prior service (credit) cost	(23.7)	(14.4)
Subtotal	190.0	117.7
Adjustments for amounts included in regulatory assets	(187.1)	(115.5)
Total	2.9	2.2
Pension Plans | Spire Missouri
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	153.1	102.8
Prior service (credit) cost	1.7	6.4
Subtotal	154.8	109.2
Adjustments for amounts included in regulatory assets	(151.9)	(107.0)
Total	2.9	2.2
Pension Plans | Spire Alabama
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	61.9	36.4
Prior service (credit) cost	(23.5)	(18.8)
Subtotal	38.4	17.6
Adjustments for amounts included in regulatory assets	(38.4)	(17.6)
Total	0.0	0.0
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	(56.3)	(44.4)
Prior service (credit) cost	(1.0)	(6.6)
Subtotal	(57.3)	(51.0)
Adjustments for amounts included in regulatory assets	57.3	51.0
Other Postretirement Plans | Spire Missouri
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	(52.6)	(35.7)
Prior service (credit) cost	0.6	(4.0)
Subtotal	(52.0)	(39.7)
Adjustments for amounts included in regulatory assets	52.0	39.7
Other Postretirement Plans | Spire Alabama
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	(2.6)	(8.0)
Prior service (credit) cost	(1.6)	(2.6)
Subtotal	(4.2)	(10.6)
Adjustments for amounts included in regulatory assets	$ 4.2	$ 10.6