FAIR VALUE MEASUREMENTS (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	$ (14.0)
Derivative liability	5.8
Spire Missouri
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(0.5)	$ (3.4)	$ (6.6)
Derivative asset	23.5	22.5	23.9
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(2.7)	(1.9)	(0.2)
Derivative liability	0.0	0.0	0.0
Quoted Prices in Active Markets (Level 1)
LIABILITIES
Derivative liability before netting adjustment	3.8
Quoted Prices in Active Markets (Level 1) | Spire Missouri
ASSETS
Derivative asset before netting adjustment	20.0	21.8	26.5
LIABILITIES
Derivative liability before netting adjustment	2.7	1.9	0.2
Significant Observable Inputs (Level 2)
LIABILITIES
Derivative liability before netting adjustment	15.6
Significant Observable Inputs (Level 2) | Spire Missouri
ASSETS
Derivative asset before netting adjustment	4.0	4.1	4.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0	0.0
Significant Unobservable Inputs (Level 3)
LIABILITIES
Derivative liability before netting adjustment	0.4
Significant Unobservable Inputs (Level 3) | Spire Missouri
ASSETS
Derivative asset before netting adjustment	0.0	0.0	0.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0	0.0
US stock/bond mutual funds | Spire Missouri
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	0.0	0.0	0.0
Derivative asset	23.5	22.4	21.2
US stock/bond mutual funds | Quoted Prices in Active Markets (Level 1) | Spire Missouri
ASSETS
Derivative asset before netting adjustment	19.5	18.3	17.2
US stock/bond mutual funds | Significant Observable Inputs (Level 2) | Spire Missouri
ASSETS
Derivative asset before netting adjustment	4.0	4.1	4.0
US stock/bond mutual funds | Significant Unobservable Inputs (Level 3) | Spire Missouri
ASSETS
Derivative asset before netting adjustment	0.0	0.0	0.0
NYMEX/ICE natural gas contracts
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables			(0.2)
Derivative liability			0.0
NYMEX/ICE natural gas contracts | Spire Missouri
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(0.5)	(3.4)	(6.6)
Derivative asset	0.0	0.0	2.2
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(2.7)	(1.9)
Derivative liability	0.0	0.0
NYMEX/ICE natural gas contracts | Quoted Prices in Active Markets (Level 1)
LIABILITIES
Derivative liability before netting adjustment			0.2
NYMEX/ICE natural gas contracts | Quoted Prices in Active Markets (Level 1) | Spire Missouri
ASSETS
Derivative asset before netting adjustment	0.5	3.4	8.8
LIABILITIES
Derivative liability before netting adjustment	2.7	1.9
NYMEX/ICE natural gas contracts | Significant Observable Inputs (Level 2)
LIABILITIES
Derivative liability before netting adjustment			0.0
NYMEX/ICE natural gas contracts | Significant Observable Inputs (Level 2) | Spire Missouri
ASSETS
Derivative asset before netting adjustment	0.0	0.0	0.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0
NYMEX/ICE natural gas contracts | Significant Unobservable Inputs (Level 3)
LIABILITIES
Derivative liability before netting adjustment			0.0
NYMEX/ICE natural gas contracts | Significant Unobservable Inputs (Level 3) | Spire Missouri
ASSETS
Derivative asset before netting adjustment	0.0	0.0	0.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0
Gasoline and heating oil contracts | Spire Missouri
ASSETS
Effects of Netting and Cash Margin Receivables /Payables		0.0	0.0
Derivative asset		0.1	0.5
Gasoline and heating oil contracts | Quoted Prices in Active Markets (Level 1) | Spire Missouri
ASSETS
Derivative asset before netting adjustment		0.1	0.5
Gasoline and heating oil contracts | Significant Observable Inputs (Level 2) | Spire Missouri
ASSETS
Derivative asset before netting adjustment		0.0	0.0
Gasoline and heating oil contracts | Significant Unobservable Inputs (Level 3) | Spire Missouri
ASSETS
Derivative asset before netting adjustment		0.0	0.0
Other | Interest rate swaps
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	0.0	0.0
Derivative liability	0.8	0.9
Other | Interest rate swaps | Quoted Prices in Active Markets (Level 1)
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0
Other | Interest rate swaps | Significant Observable Inputs (Level 2)
LIABILITIES
Derivative liability before netting adjustment	0.8	0.9
Other | Interest rate swaps | Significant Unobservable Inputs (Level 3)
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0
Gas Utility | US stock/bond mutual funds
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	0.0	0.0	0.0
Derivative asset	23.5	22.4	21.2
Gas Utility | US stock/bond mutual funds | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	19.5	18.3	17.2
Gas Utility | US stock/bond mutual funds | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	4.0	4.1	4.0
Gas Utility | US stock/bond mutual funds | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.0	0.0	0.0
Gas Utility | NYMEX/ICE natural gas contracts
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(0.5)	(3.4)	(6.6)
Derivative asset	0.0	0.0	2.2
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(2.7)	(1.9)	(0.2)
Derivative liability	0.0	0.0	0.0
Gas Utility | NYMEX/ICE natural gas contracts | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	0.5	3.4	8.8
LIABILITIES
Derivative liability before netting adjustment	2.7	1.9	0.2
Gas Utility | NYMEX/ICE natural gas contracts | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	0.0	0.0	0.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0	0.0
Gas Utility | NYMEX/ICE natural gas contracts | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.0	0.0	0.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0	0.0
Gas Utility | Gasoline and heating oil contracts
ASSETS
Effects of Netting and Cash Margin Receivables /Payables		0.0	0.0
Derivative asset		0.1	0.7
Gas Utility | Gasoline and heating oil contracts | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment		0.1	0.7
Gas Utility | Gasoline and heating oil contracts | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment		0.0	0.0
Gas Utility | Gasoline and heating oil contracts | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment		0.0	0.0
Gas Marketing
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(10.9)	(6.7)
Derivative asset	27.2	28.7
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables		(5.2)	(10.4)
Derivative liability		8.1	3.5
Gas Marketing | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	22.4	23.1
LIABILITIES
Derivative liability before netting adjustment		3.7	5.3
Gas Marketing | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	15.7	12.2
LIABILITIES
Derivative liability before netting adjustment		9.6	8.6
Gas Marketing | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.0	0.1
LIABILITIES
Derivative liability before netting adjustment		0.0	0.0
Gas Marketing | US stock/bond mutual funds
ASSETS
Effects of Netting and Cash Margin Receivables /Payables			(11.8)
Derivative asset			42.1
Gas Marketing | US stock/bond mutual funds | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment			27.4
Gas Marketing | US stock/bond mutual funds | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment			26.5
Gas Marketing | US stock/bond mutual funds | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment			0.0
Gas Marketing | NYMEX/ICE natural gas contracts
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(6.1)	(2.1)	(4.9)
Derivative asset	0.3	0.5	0.3
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(7.0)	(2.1)	(9.9)
Derivative liability	0.0	0.0	0.0
Gas Marketing | NYMEX/ICE natural gas contracts | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	2.4	1.3	0.7
LIABILITIES
Derivative liability before netting adjustment	1.1	1.8	5.1
Gas Marketing | NYMEX/ICE natural gas contracts | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	4.0	1.3	4.5
LIABILITIES
Derivative liability before netting adjustment	5.9	0.3	4.8
Gas Marketing | NYMEX/ICE natural gas contracts | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.0	0.0	0.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0	0.0
Gas Marketing | Natural gas commodity contracts
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(4.3)	(1.2)	(0.3)
Derivative asset	3.4	5.7	9.5
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(4.3)	(1.2)	(0.3)
Derivative liability	5.0	7.2	3.5
Gas Marketing | Natural gas commodity contracts | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	0.0	0.0	0.0
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0	0.0
Gas Marketing | Natural gas commodity contracts | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	7.7	6.8	9.8
LIABILITIES
Derivative liability before netting adjustment	8.9	8.4	3.8
Gas Marketing | Natural gas commodity contracts | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.0	0.1	0.0
LIABILITIES
Derivative liability before netting adjustment	$ 0.4	$ 0.0	0.0
Gas Marketing | Other
ASSETS
Effects of Netting and Cash Margin Receivables /Payables			0.0
Derivative asset			8.2
Gas Marketing | Other | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment			0.0
Gas Marketing | Other | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment			8.2
Gas Marketing | Other | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment			$ 0.0