Income Taxes - Significant Items in Net Deferred Tax Liability (Details) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020
Spire
Deferred tax assets:
Reserves not currently deductible	$ 18.3	$ 24.8
Pension and other postretirement benefits	77.4	108.3
Goodwill	0.0	0.0
Operating losses	264.5	170.4
Regulatory amount due to customers, net	33.2	36.1
Other	32.4	44.7
Deferred tax assets	425.8	384.3
Less: Valuation allowance	(0.5)	(0.9)
Total deferred tax assets	425.3	383.4
Deferred tax liabilities:
Relating to property	(693.9)	(614.0)
Regulatory pension and other postretirement benefits	(95.6)	(138.4)
Deferred gas costs	(81.1)	0.0
Other	(167.0)	(142.4)
Total deferred tax liabilities	(1,037.6)	(894.8)
Net deferred tax (liability)	(612.3)	(511.4)
Spire Missouri
Deferred tax assets:
Reserves not currently deductible	8.5	16.7
Pension and other postretirement benefits	53.4	78.5
Goodwill	0.0	0.0
Operating losses	110.3	36.0
Regulatory amount due to customers, net	29.4	32.3
Other	0.0	0.0
Deferred tax assets	201.6	163.5
Less: Valuation allowance	(0.4)	(0.9)
Total deferred tax assets	201.2	162.6
Deferred tax liabilities:
Relating to property	(464.0)	(427.1)
Regulatory pension and other postretirement benefits	(71.2)	(107.4)
Deferred gas costs	(79.5)	0.0
Other	(66.5)	(62.8)
Total deferred tax liabilities	(681.2)	(597.3)
Net deferred tax (liability)	(480.0)	(434.7)
Spire Alabama
Deferred tax assets:
Reserves not currently deductible	6.6	5.9
Pension and other postretirement benefits	0.0	0.0
Goodwill	87.2	101.3
Operating losses	130.3	111.3
Regulatory amount due to customers, net	0.0	0.0
Other	0.0	0.0
Deferred tax assets	224.1	218.5
Less: Valuation allowance	0.0	0.0
Total deferred tax assets	224.1	218.5
Deferred tax liabilities:
Relating to property	(182.7)	(151.4)
Regulatory pension and other postretirement benefits	(1.6)	(3.4)
Deferred gas costs	0.0	0.0
Other	(5.6)	(4.4)
Total deferred tax liabilities	(189.9)	(159.2)
Net deferred tax asset	$ 34.2	$ 59.3