STOCKHOLDERS' EQUITY - Narrative (Details) $ / shares in Units, equity_unit in Thousands				12 Months Ended
	Sep. 15, 2017 USD ($)	Apr. 03, 2017 $ / shares shares	May 17, 2016 USD ($) shares	Sep. 30, 2017 USD ($) shares	Sep. 30, 2016 USD ($) shares	Sep. 30, 2015 USD ($)	Dec. 31, 2014 USD ($) equity_unit	May 30, 2017 USD ($)	Nov. 11, 2016 shares	Feb. 10, 2016 USD ($)	Jun. 30, 2014 $ / shares	Jun. 20, 2014 shares
Accumulated other comprehensive income (loss), net of income taxes [Line Items]
Number of equity units issued (in shares) | equity_unit							2,875
Stated value per Corporate Unit (in dollars per share) | $ / shares											$ 50,000,000
Interest rate on Subordinated borrowing							2.00%
Issuance of stock (in shares)			2,185,000
SEC effective shelf registration Form S-3 sale and issuance authorization under the dividend reinvestment and stock purchase program (in shares)												250,000
SEC effective shelf registration Form S-3 remaining and available for issuance under the dividend reinvestment and stock purchase program (in shares)				244,130					239,945
Issuance of common stock | $			$ 133,200,000	$ 145,500,000	$ 133,200,000
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 | $				$ 1,010,800,000	$ 916,800,000
Sale of Stock, Number of Shares Issued in Transaction		2,504,684
Sale of Stock, Price Per Share | $ / shares		$ 57.3921
Share Purchase Obligation, Conversion to Common Stock, Rate Per Share		0.8712
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 | $				$ 300,000,000				$ 300,000,000.0		$ 300,000,000.0
Proceeds from Issuance of Debt | $	$ 170,000,000.0
Regulatory Preferred Stock and Debt Securities Issuance, Remaining Authorized Amount | $				$ 130,000,000
Spire Missouri | Affiliated Entity
Accumulated other comprehensive income (loss), net of income taxes [Line Items]
Issuance of stock (in shares)					0
Spire Alabama
Accumulated other comprehensive income (loss), net of income taxes [Line Items]
Issuance of common stock | $						$ (27,000,000)
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
Regulatory preferred stock and debt securities issuance, amount authorized | $				$ 75,000,000.0
Junior Subordinated Debt
Accumulated other comprehensive income (loss), net of income taxes [Line Items]
Proceeds from convertible debt | $							$ 143,800,000