SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Changes in Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Goodwill [Line Items]
Accruals for capital expenditures	$ 41.0	$ 30.4	$ 13.4
Goodwill [Roll Forward]
Beginning balance	1,164.9	946.0	937.8
Goodwill	6.7		8.2
Acquisition		218.9
Ending balance	1,171.6	1,164.9	946.0
Operating Segments | Gas Utility:
Goodwill [Roll Forward]
Beginning balance	210.2	210.2	210.2
Goodwill	0.0		0.0
Acquisition		0.0
Ending balance	210.2	210.2	210.2
Operating Segments | Gas Marketing:
Goodwill [Roll Forward]
Beginning balance	0.0	0.0	0.0
Goodwill	0.0		0.0
Acquisition		0.0
Ending balance	0.0	0.0	0.0
Other
Goodwill [Roll Forward]
Beginning balance	954.7	735.8	727.6
Goodwill	6.7		8.2
Acquisition		218.9
Ending balance	961.4	954.7	735.8
Spire Missouri
Goodwill [Line Items]
Accruals for capital expenditures	28.9	14.8	$ 9.6
Goodwill [Roll Forward]
Beginning balance	210.2
Ending balance	$ 210.2	$ 210.2