FINANCING ARRANGEMENTS AND LONG-TERM DEBT (Details)	Dec. 21, 2016 USD ($)	Dec. 14, 2016 USD ($) bank	Jan. 12, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 01, 2017 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Line of Credit Facility [Line Items]
Debt covenant, short-term debt to capital requirement, maximum		70.00%
Notes outstanding				$ 583,600,000		$ 477,300,000	$ 0
Commercial Paper Notes
Line of Credit Facility [Line Items]
Face amount	$ 975,000,000.0
Debt term	365 days
Spire Missouri
Line of Credit Facility [Line Items]
Notes outstanding				275,600,000
Spire Alabama
Line of Credit Facility [Line Items]
Notes outstanding				$ 163,100,000
Line of Credit | Revolving Credit Facility
Line of Credit Facility [Line Items]
Number of banks in loan agreement | bank		11
Maximum borrowing capacity		$ 975,000,000.0
Line of Credit | Revolving Credit Facility | Spire Missouri
Line of Credit Facility [Line Items]
Maximum borrowing capacity		475,000,000.0
Ratio of short-term debt to total capital				0.50
Line of Credit | Revolving Credit Facility | Spire Alabama
Line of Credit Facility [Line Items]
Maximum borrowing capacity		200,000,000.0
Ratio of short-term debt to total capital				0.35
Line of Credit | Revolving Credit Facility | Spire
Line of Credit Facility [Line Items]
Maximum borrowing capacity		$ 300,000,000.0
Ratio of short-term debt to total capital				0.57
Credit facility borrowings				$ 0		$ 0	$ 193,500,000
Senior Notes | Spire Alabama | Series 2017A senior notes due 2058
Line of Credit Facility [Line Items]
Face amount					$ 30,000,000
Stated interest rate					4.02%
Senior Notes | Spire Alabama | Series 2017B senior notes due 2048 | Subsequent Event
Line of Credit Facility [Line Items]
Face amount			$ 45,000,000
Stated interest rate			3.92%