FAIR VALUE MEASUREMENTS (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Jun. 30, 2018	Sep. 30, 2017	Jun. 30, 2017
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	$ (6.3)		$ (5.6)
Derivative asset	36.7		29.9
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(10.1)	$ (5.2)	(5.6)
Derivative liability	4.7	8.1	4.4
Spire Missouri
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(1.7)	(3.4)	(1.3)
Derivative asset	23.0	22.5	22.8
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(0.2)	(1.9)	(1.3)
Derivative liability	0.0	0.0	0.0
Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	20.8		21.5
LIABILITIES
Derivative liability before netting adjustment	1.1	3.7	2.1
Quoted Prices in Active Markets (Level 1) | Spire Missouri
ASSETS
Derivative asset before netting adjustment	20.6	21.8	20.0
LIABILITIES
Derivative liability before netting adjustment	0.2	1.9	1.3
Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	22.2		13.4
LIABILITIES
Derivative liability before netting adjustment	13.7	9.6	7.9
Significant Observable Inputs (Level 2) | Spire Missouri
ASSETS
Derivative asset before netting adjustment	4.1	4.1	4.1
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0	0.0
Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.0		0.6
LIABILITIES
Derivative liability before netting adjustment	0.0	0.0	0.0
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	23.0	22.4	22.1
US stock/bond mutual funds | Quoted Prices in Active Markets (Level 1) | Spire Missouri
ASSETS
Derivative asset before netting adjustment	18.9	18.3	18.0
US stock/bond mutual funds | Significant Observable Inputs (Level 2) | Spire Missouri
ASSETS
Derivative asset before netting adjustment	4.1	4.1	4.1
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 | Spire Missouri
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(1.7)	(3.4)	(1.3)
Derivative asset	0.0	0.0	0.6
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(0.2)	(1.9)	(1.3)
Derivative liability	0.0	0.0	0.0
NYMEX/ICE natural gas contracts | Quoted Prices in Active Markets (Level 1) | Spire Missouri
ASSETS
Derivative asset before netting adjustment	1.7	3.4	1.9
LIABILITIES
Derivative liability before netting adjustment	0.2	1.9	1.3
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	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	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	0.1
Gasoline and heating oil contracts | Quoted Prices in Active Markets (Level 1) | Spire Missouri
ASSETS
Derivative asset before netting adjustment		0.1	0.1
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
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	0.0
Derivative asset	1.6
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables		0.0	0.0
Derivative liability		0.9	(0.6)
Other | Interest rate swaps | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	0.0
LIABILITIES
Derivative liability before netting adjustment		0.0	0.0
Other | Interest rate swaps | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	1.6
LIABILITIES
Derivative liability before netting adjustment		0.9	(0.6)
Other | Interest rate swaps | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.0
LIABILITIES
Derivative liability before netting adjustment		0.0	0.0
Gas Utility | Operating Segments | US stock/bond mutual funds
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	0.0	0.0	0.0
Derivative asset	23.0	22.4	22.1
Gas Utility | Operating Segments | US stock/bond mutual funds | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	18.9	18.3	18.0
Gas Utility | Operating Segments | US stock/bond mutual funds | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	4.1	4.1	4.1
Gas Utility | Operating Segments | US stock/bond mutual funds | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.0	0.0	0.0
Gas Utility | Operating Segments | NYMEX/ICE natural gas contracts
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(1.7)	(3.4)	(1.3)
Derivative asset	0.0	0.0	0.6
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(0.2)	(1.9)	(1.3)
Derivative liability	0.0	0.0	0.0
Gas Utility | Operating Segments | NYMEX/ICE natural gas contracts | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	1.7	3.4	1.9
LIABILITIES
Derivative liability before netting adjustment	0.2	1.9	1.3
Gas Utility | Operating Segments | 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 | Operating Segments | 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 | Operating Segments | Gasoline and heating oil contracts
ASSETS
Effects of Netting and Cash Margin Receivables /Payables		0.0	0.0
Derivative asset		0.1	0.1
Gas Utility | Operating Segments | Gasoline and heating oil contracts | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment		0.1	0.1
Gas Utility | Operating Segments | Gasoline and heating oil contracts | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment		0.0	0.0
Gas Utility | Operating Segments | Gasoline and heating oil contracts | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment		0.0	0.0
Gas Marketing | Operating Segments
ASSETS
Effects of Netting and Cash Margin Receivables /Payables		(6.7)
Derivative asset		28.7
Gas Marketing | Operating Segments | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment		23.1
Gas Marketing | Operating Segments | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment		12.2
Gas Marketing | Operating Segments | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment		0.1
Gas Marketing | Operating Segments | NYMEX/ICE natural gas contracts
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(2.6)	(2.1)	(2.6)
Derivative asset	0.0	0.5	2.1
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(7.9)	(2.1)	(2.6)
Derivative liability	0.0	0.0	0.0
Gas Marketing | Operating Segments | NYMEX/ICE natural gas contracts | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before netting adjustment	0.2	1.3	1.5
LIABILITIES
Derivative liability before netting adjustment	0.9	1.8	0.8
Gas Marketing | Operating Segments | NYMEX/ICE natural gas contracts | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	2.4	1.3	3.2
LIABILITIES
Derivative liability before netting adjustment	7.0	0.3	1.8
Gas Marketing | Operating Segments | 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 | Operating Segments | Natural gas commodity contracts
ASSETS
Effects of Netting and Cash Margin Receivables /Payables	(2.0)	(1.2)	(1.7)
Derivative asset	12.1	5.7	5.0
LIABILITIES
Effects of Netting and Cash Margin Receivables /Payables	(2.0)	(1.2)	(1.7)
Derivative liability	4.7	7.2	5.0
Gas Marketing | Operating Segments | 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 | Operating Segments | Natural gas commodity contracts | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before netting adjustment	14.1	6.8	6.1
LIABILITIES
Derivative liability before netting adjustment	6.7	8.4	6.7
Gas Marketing | Operating Segments | Natural gas commodity contracts | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before netting adjustment	0.0	0.1	0.6
LIABILITIES
Derivative liability before netting adjustment	$ 0.0	$ 0.0	$ 0.0