INCOME TAXES - Significant Items in Net Deferred Tax Liability (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017
Deferred tax assets:
Reserves not currently deductible	$ 25.9	$ 31.5
Pension and other postretirement benefits	75.6	58.6
Operating losses	162.7	169.6
Regulatory amount due to customers, net	41.8	0.0
Other	8.2	26.0
Deferred tax assets	314.2	285.7
Less: valuation allowance	1.4	0.5
Total deferred tax assets	312.8	285.2
Deferred tax liabilities:
Relating to property	518.3	728.3
Regulatory pension and other postretirement benefits	117.1	108.0
Deferred gas costs	2.3	30.6
Other	110.9	125.8
Total deferred tax liabilities	748.6	992.7
Net deferred tax liability	435.8	707.5
Spire Missouri
Deferred tax assets:
Reserves not currently deductible	20.0	25.3
Pension and other postretirement benefits	71.9	52.7
Deferred gas costs	0.5	0.0
Operating losses	65.2	52.0
Regulatory amount due to customers, net	38.0	0.0
Deferred tax assets	195.6	130.0
Less: valuation allowance	1.4	0.5
Total deferred tax assets	194.2	129.5
Deferred tax liabilities:
Relating to property	372.6	563.2
Regulatory pension and other postretirement benefits	113.4	108.0
Deferred gas costs	0.0	25.0
Other	69.2	57.1
Total deferred tax liabilities	555.2	753.3
Net deferred tax liability	361.0	623.8
Spire Alabama
Deferred tax assets:
Reserves not currently deductible	5.3	6.0
Pension and other postretirement benefits	0.0	4.4
Goodwill	129.6	214.4
Operating losses	86.0	88.3
Deferred tax assets	220.9	313.1
Deferred tax liabilities:
Relating to property	111.9	119.3
Regulatory pension and other postretirement benefits	1.8	0.0
Other	5.4	8.2
Total deferred tax liabilities	119.1	127.5
Net deferred tax asset	$ 101.8	$ 185.6