INCOME TAXES - Significant Items in Net Deferred Tax Liability (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
Deferred tax assets:
Reserves not currently deductible	$ 31.5	$ 21.3
Pension and other postretirement benefits	58.6	68.3
Operating losses	169.6	102.3
Other	26.0	0.0
Deferred tax assets	285.7	191.9
Less: valuation allowance	0.5	0.9
Total deferred tax assets	285.2	191.0
Deferred tax liabilities:
Relating to property	728.3	623.1
Regulatory pension and other postretirement benefits	108.0	106.8
Deferred gas costs	30.6	20.0
Other	125.8	48.4
Total deferred tax liabilities	992.7	798.3
Net deferred tax liability	707.5	607.3
Spire Missouri
Deferred tax assets:
Reserves not currently deductible		14.9
Pension and other postretirement benefits		56.9
Operating losses		29.9
Deferred tax assets	130.0	101.7
Less: valuation allowance	0.5	0.9
Total deferred tax assets	129.5	100.8
Deferred tax liabilities:
Relating to property		497.0
Regulatory pension and other postretirement benefits		106.8
Deferred gas costs		20.0
Other		33.9
Total deferred tax liabilities	753.3	657.7
Net deferred tax liability	623.8	556.9
Spire Alabama
Deferred tax assets:
Reserves not currently deductible	6.0	6.3
Pension and other postretirement benefits	4.4	11.4
Goodwill	214.4	233.4
Operating losses	88.3	60.2
Deferred tax assets	313.1	311.3
Deferred tax liabilities:
Relating to property	119.3	87.6
Other	8.2	2.3
Total deferred tax liabilities	127.5	89.9
Net deferred tax asset	$ 185.6	$ 221.4