INCOME TAXES - Net Provisions for Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Federal
Current	$ 0.1	$ 0.1	$ (3.3)
Deferred	67.7	62.0	58.8
Investment tax credits	(0.2)	(0.2)	(0.2)
State and local
Current	0.5	0.6	0.0
Deferred	9.5	7.0	6.9
Total income tax expense	77.6	69.5	62.2
Spire Missouri
Federal
Current	0.0	0.0	(2.1)
Deferred	42.0	37.5	40.9
Investment tax credits	(0.2)	(0.2)	(0.2)
State and local
Current	0.0	0.1	(0.1)
Deferred	5.7	8.0	4.7
Total income tax expense	47.5	45.4	43.2
Spire Alabama
Federal
Current	0.0	(0.8)	0.0
Deferred	31.6	29.4	25.9
State and local
Current	0.0	0.0	0.1
Deferred	4.2	3.8	3.3
Total income tax expense	$ 35.8	$ 32.4	$ 29.3