LONG-TERM DEBT - Narrative (Details) $ / shares in Units, equity_unit in Thousands					12 Months Ended
	Sep. 15, 2017 USD ($)	Apr. 03, 2017 USD ($) equity_unit $ / shares shares	Feb. 22, 2017 USD ($)	Jun. 11, 2014 USD ($) $ / shares	Sep. 30, 2017 USD ($)	Dec. 31, 2014 USD ($) equity_unit	Sep. 30, 2014 equity_unit	May 30, 2017 USD ($)	Mar. 15, 2017 USD ($)	Mar. 10, 2017 USD ($)	Sep. 30, 2016	Feb. 10, 2016 USD ($)	Jun. 30, 2014 $ / shares
Debt Instrument [Line Items]
Long-term Debt, Gross					$ 2,112,000,000
Interest rate on Subordinated borrowing						2.00%
Number of equity units issued (in shares) | equity_unit						2,875
Stated value per Corporate Unit (in dollars per share) | $ / shares													$ 50,000,000
Percent of common stock equity					48.70%						46.10%
Percent of long-term debt					51.30%						53.90%
Sale of Stock, Number of Shares Issued in Transaction | shares		2,504,684
Sale of Stock, Price Per Share | $ / shares		$ 57.3921
Payment to Purchase Share, Price Per Share | $ / shares		$ 67.50
Share Purchase Obligation, Conversion to Common Stock, Rate Per Share		0.8712
Repayments of Short-term Debt		$ 142,000,000
Senior Notes
Debt Instrument [Line Items]
Long-term debt issued amount			$ 6,200,000
Proceeds from Issuance of Debt			150,000,000
Senior Notes | Senior Notes Due March 15, 2027
Debt Instrument [Line Items]
Long-term debt issued amount									$ 100,000,000.0
Stated interest rate on debt issued									3.93%
Senior Notes | Senior Notes Due in 2024
Debt Instrument [Line Items]
Long-term debt issued amount			$ 143,800,000.0
Stated interest rate on debt issued			3.543%
Senior Notes | One to six months prior to maturity | Subject to Callable at Par
Debt Instrument [Line Items]
Redemption period prior to maturity, minimum					1 month
Redemption period prior to maturity, maximum					6 months
Bonds | Bond Due September Fifteen Twenty Thirty Two [Member]
Debt Instrument [Line Items]
Long-term debt issued amount	$ 50,000,000.0
Stated interest rate on debt issued	3.68%
Bonds | Bond Due September Fifteen Twenty Forty Seven [Member]
Debt Instrument [Line Items]
Long-term debt issued amount	$ 70,000,000.0
Stated interest rate on debt issued	4.23%
Bonds | Bond Due September Fifteen Twenty Fifty Seven [Member]
Debt Instrument [Line Items]
Long-term debt issued amount	$ 50,000,000.0
Stated interest rate on debt issued	4.38%
Junior Subordinated Debt
Debt Instrument [Line Items]
Proceeds from convertible debt						$ 143,800,000
Notes Payable to Banks
Debt Instrument [Line Items]
Debt Instrument, Repurchase Amount										$ 250,000,000.0
Other subsidiaries
Debt Instrument [Line Items]
Long-term Debt, Gross					$ 67,000,000
Spire Missouri
Debt Instrument [Line Items]
Long-term Debt, Gross					980,000,000
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
Long-term debt					$ 975,000,000
Percent of common stock equity					54.60%						57.10%
Percent of long-term debt					45.40%						42.90%
Spire Missouri | No Call Options
Debt Instrument [Line Items]
Long-term Debt, Gross					$ 25,000,000
Long-term debt					25,000,000
Spire Missouri | Make Whole Call Options
Debt Instrument [Line Items]
Long-term Debt, Gross					1,037,000,000
Long-term debt					435,000,000
Spire Missouri | Subject to Callable Currently [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross					5,000,000
Spire Missouri | Subject to Callable at Par
Debt Instrument [Line Items]
Long-term Debt, Gross					1,045,000,000
Long-term debt					$ 520,000,000
Spire Missouri | Three to six months prior to maturity | Subject to Callable at Par
Debt Instrument [Line Items]
Redemption period prior to maturity, minimum					3 months
Redemption period prior to maturity, maximum					6 months
Spire Alabama
Debt Instrument [Line Items]
Long-term Debt, Gross					$ 250,000,000
Regulatory preferred stock and debt securities issuance, amount authorized					75,000,000.0
Long-term debt					$ 250,000,000
Percent of common stock equity					77.80%						77.80%
Percent of long-term debt					22.20%						22.20%
Spire Alabama | Make Whole Call Options
Debt Instrument [Line Items]
Long-term debt					$ 250,000,000
Spire Alabama
Debt Instrument [Line Items]
Interest rate on Subordinated borrowing				2.00%
Number of equity units issued (in shares) | equity_unit		2,875					2,875
Stated value per Corporate Unit (in dollars per share) | $ / shares				$ 50
Spire Alabama | Junior Subordinated Debt
Debt Instrument [Line Items]
Proceeds from convertible debt				$ 143,800,000