INCOME TAXES - Significant Items in Net Deferred Tax Liability (Details) - USD ($) $ in Millions	Sep. 30, 2016	Sep. 30, 2015
Deferred tax assets:
Reserves not currently deductible	$ 21.3	$ 14.8
Pension and other postretirement benefits	68.3	62.5
Operating losses	102.3	47.3
Other	0.0	1.5
Deferred tax assets	191.9	126.1
Less: valuation allowance	0.9	0.0
Total deferred tax assets	191.0	126.1
Deferred tax liabilities:
Relating to property	623.1	472.1
Regulatory pension and other postretirement benefits	106.8	110.6
Deferred gas costs	20.0	8.1
Other	48.4	11.6
Total deferred tax liabilities	798.3	602.4
Net deferred tax liability	607.3	476.3
Net deferred tax asset – current	0.0	5.8
Net deferred tax liability	607.3
Det deferred tax asset (liability) - noncurrent		482.1
Laclede Gas
Deferred tax assets:
Reserves not currently deductible	14.9	15.4
Pension and other postretirement benefits	56.9	62.5
Operating losses	29.9	3.7
Other	0.0	1.5
Deferred tax assets	101.7	83.1
Less: valuation allowance	0.9	0.0
Total deferred tax assets	100.8	83.1
Deferred tax liabilities:
Relating to property	497.0	425.0
Regulatory pension and other postretirement benefits	106.8	120.2
Deferred gas costs	20.0	8.2
Other	33.9	14.5
Total deferred tax liabilities	657.7	567.9
Net deferred tax liability	556.9	484.8
Net deferred tax asset – current	0.0	0.4
Net deferred tax liability	556.9
Det deferred tax asset (liability) - noncurrent		485.2
Alagasco
Deferred tax assets:
Reserves not currently deductible	6.3	7.0
Pension and other postretirement benefits	11.4	9.6
Goodwill	233.4	251.5
Operating losses	60.2	32.4
Other	0.0	1.4
Deferred tax assets	311.3	301.9
Deferred tax liabilities:
Relating to property	87.6	45.1
Other	2.3	2.2
Total deferred tax liabilities	89.9	47.3
Net deferred tax asset	221.4	254.6
Net deferred tax asset – current	0.0	6.2
Deferred income tax	$ 221.4
Deferred income tax		$ 248.4