Income Taxes - Significant Items in Net Deferred Tax Liability (Details) - USD ($) $ in Millions	Sep. 30, 2019	Sep. 30, 2018
Spire
Deferred tax assets:
Reserves not currently deductible	$ 31.4	$ 25.9
Pension and other postretirement benefits	102.7	75.6
Goodwill	0.0	0.0
Operating losses	162.2	162.7
Regulatory amount due to customers, net	46.5	41.8
Other	20.0	8.2
Deferred tax assets	362.8	314.2
Less: Valuation allowance	(1.8)	(1.4)
Total deferred tax assets	361.0	312.8
Deferred tax liabilities:
Relating to property	(562.9)	(518.3)
Regulatory pension and other postretirement benefits	(135.9)	(117.1)
Other	(113.6)	(113.2)
Total deferred tax liabilities	(812.4)	(748.6)
Net deferred tax (liability)	(451.4)	(435.8)
Total deferred tax assets	361.0	312.8
Total deferred tax liabilities	(812.4)	(748.6)
Spire Missouri
Deferred tax assets:
Reserves not currently deductible	21.2	20.0
Pension and other postretirement benefits	78.7	71.9
Goodwill	0.0	0.0
Operating losses	51.1	65.2
Regulatory amount due to customers, net	42.7	38.0
Other	1.2	0.5
Deferred tax assets	194.9	195.6
Less: Valuation allowance	(1.8)	(1.4)
Total deferred tax assets	193.1	194.2
Deferred tax liabilities:
Relating to property	(395.1)	(372.6)
Regulatory pension and other postretirement benefits	(111.7)	(113.4)
Other	(50.9)	(69.2)
Total deferred tax liabilities	(557.7)	(555.2)
Net deferred tax (liability)	(364.6)	(361.0)
Total deferred tax assets	193.1	194.2
Total deferred tax liabilities	(557.7)	(555.2)
Spire Alabama
Deferred tax assets:
Reserves not currently deductible	6.2	5.3
Pension and other postretirement benefits	0.0	0.0
Goodwill	116.8	129.6
Operating losses	94.2	86.0
Regulatory amount due to customers, net	0.0	0.0
Other	0.0	0.0
Deferred tax assets	217.2	220.9
Less: Valuation allowance	0.0	0.0
Total deferred tax assets	217.2	220.9
Deferred tax liabilities:
Relating to property	(129.3)	(111.9)
Regulatory pension and other postretirement benefits	(2.0)	(1.8)
Other	(4.6)	(5.4)
Total deferred tax liabilities	(135.9)	(119.1)
Total deferred tax assets	217.2	220.9
Total deferred tax liabilities	(135.9)	(119.1)
Net deferred tax asset	$ 81.3	$ 101.8