SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Business Combinations (Details) - USD ($)		1 Months Ended		12 Months Ended
	Sep. 12, 2016	May 31, 2018	Dec. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Business Acquisition [Line Items]
Working capital settlement payment				$ 0	$ (3,800,000)	$ 317,700,000
Total Operating Revenues				1,965,000,000	1,740,700,000	1,632,400,000
Net Income				$ 214,200,000	$ 161,600,000	$ 153,900,000
Basic Earnings Per Share of Common Stock (in dollars per share)				$ 4.35	$ 3.44	$ 3.48
Diluted Earnings Per Share of Common Stock (in dollars per share)				$ 4.33	$ 3.43	$ 3.46
Spire EnergySouth Inc
Business Acquisition [Line Items]
Percentage of voting interest acquired	100.00%
Working capital settlement payment	$ 313,900,000
Total Operating Revenues				$ 102,100,000	$ 95,500,000	$ 3,300,000
Net Income				$ 13,800,000	$ 9,400,000	$ (200,000)
Natural Gas Storage Facility
Business Acquisition [Line Items]
Percentage of voting interest acquired		100.00%	80.00%	80.00%
Consideration		$ 12,200,000	$ 24,800,000.0
Payments to acquire business			16,000,000.0
Non-interest-bearing note			10,000,000.0
Non-interest-bearing note, value			$ 8,800,000