Fair Value Measurements - Schedule of Fair Value Assets and Liabilities Measured on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Jun. 30, 2019	Sep. 30, 2018	Jun. 30, 2018
ASSETS
Effects of netting and cash margin receivables/payables	$ (7.9)	$ (8.4)	$ (6.3)
Derivative asset after the effects of netting	56.9	48.2	36.7
LIABILITIES
Effects of netting and cash margin receivables/payables	(19.6)	(14.8)	(10.1)
Derivative liability after the effects of netting	40.4	6.2	4.7
Spire Missouri
ASSETS
Effects of netting and cash margin receivables/payables		(2.7)	(1.7)
Derivative asset after the effects of netting		20.3	23.0
U.S. stock/bond mutual funds \| Spire Missouri
ASSETS
Effects of netting and cash margin receivables/payables	0.0	0.0	0.0
Derivative asset after the effects of netting	20.4	20.3	23.0
NYMEX/ICE natural gas contracts \| Spire Missouri
ASSETS
Effects of netting and cash margin receivables/payables		(2.7)	(1.7)
Derivative asset after the effects of netting		0.0	0.0
LIABILITIES
Effects of netting and cash margin receivables/payables	(11.1)	(1.9)	(0.2)
Derivative liability after the effects of netting	0.0	0.0	0.0
Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before effects of netting	37.8	32.1	20.8
LIABILITIES
Derivative liability before effects of netting	11.8	2.8	1.1
Quoted Prices in Active Markets (Level 1) \| Spire Missouri
ASSETS
Derivative asset before effects of netting		23.0	20.6
Quoted Prices in Active Markets (Level 1) \| U.S. stock/bond mutual funds \| Spire Missouri
ASSETS
Derivative asset before effects of netting	20.4	20.3	18.9
Quoted Prices in Active Markets (Level 1) \| NYMEX/ICE natural gas contracts \| Spire Missouri
ASSETS
Derivative asset before effects of netting		2.7	1.7
LIABILITIES
Derivative liability before effects of netting	11.1	1.9	0.2
Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before effects of netting	27.0	24.5	22.2
LIABILITIES
Derivative liability before effects of netting	48.0	18.0	13.7
Significant Observable Inputs (Level 2) \| Spire Missouri
ASSETS
Derivative asset before effects of netting		0.0	4.1
Significant Observable Inputs (Level 2) \| U.S. stock/bond mutual funds \| Spire Missouri
ASSETS
Derivative asset before effects of netting	0.0	0.0	4.1
Significant Observable Inputs (Level 2) \| NYMEX/ICE natural gas contracts \| Spire Missouri
ASSETS
Derivative asset before effects of netting		0.0	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0	0.0
Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before effects of netting	0.0	0.0	0.0
LIABILITIES
Derivative liability before effects of netting	0.2	0.2	0.0
Significant Unobservable Inputs (Level 3) \| Spire Missouri
ASSETS
Derivative asset before effects of netting		0.0	0.0
Significant Unobservable Inputs (Level 3) \| U.S. stock/bond mutual funds \| Spire Missouri
ASSETS
Derivative asset before effects of netting	0.0	0.0	0.0
Significant Unobservable Inputs (Level 3) \| NYMEX/ICE natural gas contracts \| Spire Missouri
ASSETS
Derivative asset before effects of netting		0.0	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0	0.0
Corporate, Non-Segment \| U.S. stock/bond mutual funds
ASSETS
Effects of netting and cash margin receivables/payables	0.0	0.0
Derivative asset after the effects of netting	16.6	8.9
Corporate, Non-Segment \| Interest rate swaps
ASSETS
Effects of netting and cash margin receivables/payables		0.0	0.0
Derivative asset after the effects of netting		3.0	1.6
LIABILITIES
Effects of netting and cash margin receivables/payables	0.0
Derivative liability after the effects of netting	27.4
Corporate, Non-Segment \| Quoted Prices in Active Markets (Level 1) \| U.S. stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	16.6	8.9
Corporate, Non-Segment \| Quoted Prices in Active Markets (Level 1) \| Interest rate swaps
ASSETS
Derivative asset before effects of netting		0.0	0.0
LIABILITIES
Derivative liability before effects of netting	0.0
Corporate, Non-Segment \| Significant Observable Inputs (Level 2) \| U.S. stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	0.0	0.0
Corporate, Non-Segment \| Significant Observable Inputs (Level 2) \| Interest rate swaps
ASSETS
Derivative asset before effects of netting		3.0	1.6
LIABILITIES
Derivative liability before effects of netting	27.4
Corporate, Non-Segment \| Significant Unobservable Inputs (Level 3) \| U.S. stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	0.0	0.0
Corporate, Non-Segment \| Significant Unobservable Inputs (Level 3) \| Interest rate swaps
ASSETS
Derivative asset before effects of netting		0.0	0.0
LIABILITIES
Derivative liability before effects of netting	0.0
Gas Utility \| Operating Segments \| U.S. stock/bond mutual funds
ASSETS
Effects of netting and cash margin receivables/payables	0.0	0.0	0.0
Derivative asset after the effects of netting	20.4	20.3	23.0
Gas Utility \| Operating Segments \| NYMEX/ICE natural gas contracts
ASSETS
Effects of netting and cash margin receivables/payables		(2.7)	(1.7)
Derivative asset after the effects of netting		0.0	0.0
LIABILITIES
Effects of netting and cash margin receivables/payables	(11.1)	(1.9)	(0.2)
Derivative liability after the effects of netting	0.0	0.0	0.0
Gas Utility \| Operating Segments \| Quoted Prices in Active Markets (Level 1) \| U.S. stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	20.4	20.3	18.9
Gas Utility \| Operating Segments \| Quoted Prices in Active Markets (Level 1) \| NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting		2.7	1.7
LIABILITIES
Derivative liability before effects of netting	11.1	1.9	0.2
Gas Utility \| Operating Segments \| Significant Observable Inputs (Level 2) \| U.S. stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	0.0	0.0	4.1
Gas Utility \| Operating Segments \| Significant Observable Inputs (Level 2) \| NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting		0.0	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0	0.0
Gas Utility \| Operating Segments \| Significant Unobservable Inputs (Level 3) \| U.S. stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	0.0	0.0	0.0
Gas Utility \| Operating Segments \| Significant Unobservable Inputs (Level 3) \| NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting		0.0	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0	0.0
Gas Marketing \| Operating Segments \| NYMEX/ICE natural gas contracts
ASSETS
Effects of netting and cash margin receivables/payables	(5.5)	(4.2)	(2.6)
Derivative asset after the effects of netting	0.3	0.0	0.0
LIABILITIES
Effects of netting and cash margin receivables/payables	(6.1)	(11.4)	(7.9)
Derivative liability after the effects of netting	0.0	0.0	0.0
Gas Marketing \| Operating Segments \| Natural gas commodity contracts
ASSETS
Effects of netting and cash margin receivables/payables	(2.4)	(1.5)	(2.0)
Derivative asset after the effects of netting	19.6	16.0	12.1
LIABILITIES
Effects of netting and cash margin receivables/payables	(2.4)	(1.5)	(2.0)
Derivative liability after the effects of netting	13.0	6.2	4.7
Gas Marketing \| Operating Segments \| Quoted Prices in Active Markets (Level 1) \| NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting	0.8	0.2	0.2
LIABILITIES
Derivative liability before effects of netting	0.7	0.9	0.9
Gas Marketing \| Operating Segments \| Quoted Prices in Active Markets (Level 1) \| Natural gas commodity contracts
ASSETS
Derivative asset before effects of netting	0.0	0.0	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0	0.0
Gas Marketing \| Operating Segments \| Significant Observable Inputs (Level 2) \| NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting	5.0	4.0	2.4
LIABILITIES
Derivative liability before effects of netting	5.4	10.5	7.0
Gas Marketing \| Operating Segments \| Significant Observable Inputs (Level 2) \| Natural gas commodity contracts
ASSETS
Derivative asset before effects of netting	22.0	17.5	14.1
LIABILITIES
Derivative liability before effects of netting	15.2	7.5	6.7
Gas Marketing \| Operating Segments \| Significant Unobservable Inputs (Level 3) \| NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting	0.0	0.0	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0	0.0
Gas Marketing \| Operating Segments \| Significant Unobservable Inputs (Level 3) \| Natural gas commodity contracts
ASSETS
Derivative asset before effects of netting	0.0	0.0	0.0
LIABILITIES
Derivative liability before effects of netting	$ 0.2	$ 0.2	$ 0.0

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