Income Taxes - Significant Items in Net Deferred Tax Liability (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Spire
Deferred tax assets:
Reserves not currently deductible	$ 24.8	$ 31.4
Pension and other postretirement benefits	108.3	102.7
Goodwill	0.0	0.0
Operating losses	170.4	162.2
Regulatory amount due to customers, net	36.1	46.5
Other	44.7	20.0
Deferred tax assets	384.3	362.8
Less: Valuation allowance	(0.9)	(1.8)
Total deferred tax assets	383.4	361.0
Deferred tax liabilities:
Relating to property	(614.0)	(562.9)
Regulatory pension and other postretirement benefits	(138.4)	(135.9)
Other	(142.4)	(113.6)
Total deferred tax liabilities	(894.8)	(812.4)
Net deferred tax (liability)	(511.4)	(451.4)
Spire Missouri
Deferred tax assets:
Reserves not currently deductible	16.7	21.2
Pension and other postretirement benefits	78.5	78.7
Goodwill	0.0	0.0
Operating losses	36.0	51.1
Regulatory amount due to customers, net	32.3	42.7
Other	0.0	1.2
Deferred tax assets	163.5	194.9
Less: Valuation allowance	(0.9)	(1.8)
Total deferred tax assets	162.6	193.1
Deferred tax liabilities:
Relating to property	(427.1)	(395.1)
Regulatory pension and other postretirement benefits	(107.4)	(111.7)
Other	(62.8)	(50.9)
Total deferred tax liabilities	(597.3)	(557.7)
Net deferred tax (liability)	(434.7)	(364.6)
Spire Alabama
Deferred tax assets:
Reserves not currently deductible	5.9	6.2
Pension and other postretirement benefits	0.0	0.0
Goodwill	101.3	116.8
Operating losses	111.3	94.2
Regulatory amount due to customers, net	0.0	0.0
Other	0.0	0.0
Deferred tax assets	218.5	217.2
Less: Valuation allowance	0.0	0.0
Total deferred tax assets	218.5	217.2
Deferred tax liabilities:
Relating to property	(151.4)	(129.3)
Regulatory pension and other postretirement benefits	(3.4)	(2.0)
Other	(4.4)	(4.6)
Total deferred tax liabilities	(159.2)	(135.9)
Net deferred tax asset	$ 59.3	$ 81.3