SHAREHOLDERS' EQUITY - Narrative (Details) - USD ($)			12 Months Ended
	Apr. 03, 2017	May 17, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2014	Nov. 12, 2018	May 10, 2018	Jun. 20, 2014
Accumulated other comprehensive income (loss), net of income taxes [Line Items]
Issuance of stock (in shares)		2,185,000
Issuance of common stock		$ 133,200,000	$ 153,000,000	$ 145,500,000	$ 133,200,000
Shares of common stock issued (in shares)	2,504,684
SEC effective shelf registration Form S-3 sale and issuance authorization under the dividend reinvestment and stock purchase program (in shares)								2,300,000	250,000
SEC effective shelf registration Form S-3 remaining and available for issuance under the dividend reinvestment and stock purchase program (in shares)			221,107					68.75
Preferred stock shares authorized (in shares)			5,000,000	5,000,000
Preferred stock shares issued (in shares)			0	0
Preferred stock shares outstanding (in shares)			0	0
Dividend payment restriction formula component - fixed amount			$ 8,000,000.0
Retained earnings available to pay dividends in period			$ 1,097,600,000	$ 1,010,800,000
Spire Missouri
Accumulated other comprehensive income (loss), net of income taxes [Line Items]
Preferred stock shares authorized (in shares)			1,480,000	1,480,000
Regulatory preferred stock and debt securities issuance, amount authorized			$ 500,000,000
Spire Alabama
Accumulated other comprehensive income (loss), net of income taxes [Line Items]
Issuance of common stock					$ (29,000,000)
Preferred stock shares authorized (in shares)			120,000	120,000
Preferred stock shares issued (in shares)			0	0
Preferred stock shares outstanding (in shares)			0	0
Subsequent Event
Accumulated other comprehensive income (loss), net of income taxes [Line Items]
SEC effective shelf registration Form S-3 remaining and available for issuance under the dividend reinvestment and stock purchase program (in shares)							216,678
Junior Subordinated Debt
Accumulated other comprehensive income (loss), net of income taxes [Line Items]
Proceeds from convertible debt						$ 142,000,000