Fair Value Measurements - Schedule of Fair Value Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018
ASSETS
Derivative asset after the effects of netting	$ 14,800	$ 19,000
LIABILITIES
Derivative liability after the effects of netting	54,800	6,200
Spire Missouri
ASSETS
Derivative asset after the effects of netting	0	0
LIABILITIES
Derivative liability after the effects of netting	0	0
Fair Value, Measurements, Recurring
ASSETS
Effects of netting and cash margin receivables/payables	(9,400)	(8,400)
Derivative asset after the effects of netting	50,800	48,200
LIABILITIES
Effects of netting and cash margin receivables/payables	(23,700)	(14,800)
Derivative liability after the effects of netting	54,800	6,200
Fair Value, Measurements, Recurring | Spire Missouri
ASSETS
Effects of netting and cash margin receivables/payables		(2,700)
Derivative asset after the effects of netting		20,300
Fair Value, Measurements, Recurring | U.S. stock/bond mutual funds | Spire Missouri
ASSETS
Effects of netting and cash margin receivables/payables	0	0
Derivative asset after the effects of netting	20,500	20,300
Fair Value, Measurements, Recurring | NYMEX/ICE natural gas contracts | Spire Missouri
ASSETS
Effects of netting and cash margin receivables/payables		(2,700)
Derivative asset after the effects of netting		0
LIABILITIES
Effects of netting and cash margin receivables/payables	(12,300)	(1,900)
Derivative liability after the effects of netting	0	0
Fair Value, Measurements, Recurring | Other | U.S. stock/bond mutual funds
ASSETS
Effects of netting and cash margin receivables/payables	0	0
Derivative asset after the effects of netting	15,500	8,900
Fair Value, Measurements, Recurring | Other | Interest Rate Swap
ASSETS
Effects of netting and cash margin receivables/payables		0
Derivative asset after the effects of netting		3,000
LIABILITIES
Effects of netting and cash margin receivables/payables	0
Derivative liability after the effects of netting	43,400
Fair Value, Measurements, Recurring | Gas Utility | Operating Segments | U.S. stock/bond mutual funds
ASSETS
Effects of netting and cash margin receivables/payables	0	0
Derivative asset after the effects of netting	20,500	20,300
Fair Value, Measurements, Recurring | Gas Utility | Operating Segments | NYMEX/ICE natural gas contracts
ASSETS
Effects of netting and cash margin receivables/payables		(2,700)
Derivative asset after the effects of netting		0
LIABILITIES
Effects of netting and cash margin receivables/payables	(12,300)	(1,900)
Derivative liability after the effects of netting	0	0
Fair Value, Measurements, Recurring | Gas Marketing | Operating Segments | NYMEX/ICE natural gas contracts
ASSETS
Effects of netting and cash margin receivables/payables	(6,900)	(4,200)
Derivative asset after the effects of netting	500	0
LIABILITIES
Effects of netting and cash margin receivables/payables	(8,900)	(11,400)
Derivative liability after the effects of netting	0	0
Fair Value, Measurements, Recurring | Gas Marketing | Operating Segments | Natural gas commodity contracts
ASSETS
Effects of netting and cash margin receivables/payables	(2,500)	(1,500)
Derivative asset after the effects of netting	14,300	16,000
LIABILITIES
Effects of netting and cash margin receivables/payables	(2,500)	(1,500)
Derivative liability after the effects of netting	11,400	6,200
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before effects of netting	36,900	32,100
LIABILITIES
Derivative liability before effects of netting	12,700	2,800
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | Spire Missouri
ASSETS
Derivative asset before effects of netting		23,000
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | U.S. stock/bond mutual funds | Spire Missouri
ASSETS
Derivative asset before effects of netting	20,500	20,300
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | NYMEX/ICE natural gas contracts | Spire Missouri
ASSETS
Derivative asset before effects of netting		2,700
LIABILITIES
Derivative liability before effects of netting	12,300	1,900
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | Other | U.S. stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	15,500	8,900
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | Other | Interest Rate Swap
ASSETS
Derivative asset before effects of netting		0
LIABILITIES
Derivative liability before effects of netting	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | Gas Utility | Operating Segments | U.S. stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	20,500	20,300
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | Gas Utility | Operating Segments | NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting		2,700
LIABILITIES
Derivative liability before effects of netting	12,300	1,900
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | Gas Marketing | Operating Segments | NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting	900	200
LIABILITIES
Derivative liability before effects of netting	400	900
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | Gas Marketing | Operating Segments | Natural gas commodity contracts
ASSETS
Derivative asset before effects of netting	0	0
LIABILITIES
Derivative liability before effects of netting	0	0
Fair Value, Measurements, Recurring | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before effects of netting	23,300	24,500
LIABILITIES
Derivative liability before effects of netting	65,700	18,000
Fair Value, Measurements, Recurring | Significant Observable Inputs (Level 2) | Spire Missouri
ASSETS
Derivative asset before effects of netting		0
Fair Value, Measurements, Recurring | Significant Observable Inputs (Level 2) | U.S. stock/bond mutual funds | Spire Missouri
ASSETS
Derivative asset before effects of netting	0	0
Fair Value, Measurements, Recurring | Significant Observable Inputs (Level 2) | NYMEX/ICE natural gas contracts | Spire Missouri
ASSETS
Derivative asset before effects of netting		0
LIABILITIES
Derivative liability before effects of netting	0	0
Fair Value, Measurements, Recurring | Significant Observable Inputs (Level 2) | Other | U.S. stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	0	0
Fair Value, Measurements, Recurring | Significant Observable Inputs (Level 2) | Other | Interest Rate Swap
ASSETS
Derivative asset before effects of netting		3,000
LIABILITIES
Derivative liability before effects of netting	43,400
Fair Value, Measurements, Recurring | Significant Observable Inputs (Level 2) | Gas Utility | Operating Segments | U.S. stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	0	0
Fair Value, Measurements, Recurring | Significant Observable Inputs (Level 2) | Gas Utility | Operating Segments | NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting		0
LIABILITIES
Derivative liability before effects of netting	0	0
Fair Value, Measurements, Recurring | Significant Observable Inputs (Level 2) | Gas Marketing | Operating Segments | NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting	6,500	4,000
LIABILITIES
Derivative liability before effects of netting	8,500	10,500
Fair Value, Measurements, Recurring | Significant Observable Inputs (Level 2) | Gas Marketing | Operating Segments | Natural gas commodity contracts
ASSETS
Derivative asset before effects of netting	16,800	17,500
LIABILITIES
Derivative liability before effects of netting	13,800	7,500
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before effects of netting	0	0
LIABILITIES
Derivative liability before effects of netting	100	200
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Spire Missouri
ASSETS
Derivative asset before effects of netting		0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | U.S. stock/bond mutual funds | Spire Missouri
ASSETS
Derivative asset before effects of netting	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | NYMEX/ICE natural gas contracts | Spire Missouri
ASSETS
Derivative asset before effects of netting		0
LIABILITIES
Derivative liability before effects of netting	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Other | U.S. stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Other | Interest Rate Swap
ASSETS
Derivative asset before effects of netting		0
LIABILITIES
Derivative liability before effects of netting	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Gas Utility | Operating Segments | U.S. stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Gas Utility | Operating Segments | NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting		0
LIABILITIES
Derivative liability before effects of netting	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Gas Marketing | Operating Segments | NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting	0	0
LIABILITIES
Derivative liability before effects of netting	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Gas Marketing | Operating Segments | Natural gas commodity contracts
ASSETS
Derivative asset before effects of netting	0	0
LIABILITIES
Derivative liability before effects of netting	$ 100	$ 200