INCOME TAXES - Unrecognized Tax Benefit Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits, beginning of year	$ 10.0	$ 7.1	$ 4.6
Increases related to tax positions taken in current year	2.4	3.4	2.9
Reductions due to lapse of applicable statute of limitations	(1.4)	(0.5)	(0.4)
Unrecognized tax benefits, end of year	11.0	10.0	7.1
Spire Missouri
Unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits, beginning of year	9.7	6.9	4.2
Increases related to tax positions taken in current year	2.4	3.3	2.9
Reductions due to lapse of applicable statute of limitations	(1.4)	(0.5)	(0.2)
Unrecognized tax benefits, end of year	$ 10.7	$ 9.7	$ 6.9