Summary of Significant Accounting Policies - Additional Information (Details)	1 Months Ended			12 Months Ended
	Oct. 31, 2018 USD ($)	May 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2019 USD ($) ReportingUnit	Sep. 30, 2019 USD ($)	Sep. 30, 2019 USD ($) operating-segment	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Oct. 01, 2019 USD ($)	Oct. 25, 2018
Summary Of Significant Accounting Policies [Line Items]
Number of reportable segments						2		2
Difference between replacement cost of gas storage and inventory basis amount					$ 17,000,000.0				$ 14,500,000
Goodwill				$ 1,171,600,000	1,171,600,000	$ 1,171,600,000	$ 1,171,600,000	$ 1,171,600,000	1,171,600,000
Goodwill accumulated impairment loss				$ 0	0	0	0	0
Number of months of the original maturity of debt instruments purchased				3 months
Restricted cash				$ 17,000,000.0	17,000,000.0	17,000,000.0	17,000,000.0	17,000,000.0
Finance receivable, net				11,700,000	11,700,000	11,700,000	11,700,000	11,700,000	13,100,000
ASU 2017-07
Summary Of Significant Accounting Policies [Line Items]
Reclassification of net periodic benefit costs (income) to other income (expense)									$ 14,400,000	$ 19,200,000
ASC 842 | Subsequent Event
Summary Of Significant Accounting Policies [Line Items]
Operating lease, right-of-use assets											$ 71,100,000
Operating lease, liability											71,100,000
Gas Utility
Summary Of Significant Accounting Policies [Line Items]
Goodwill				210,200,000	210,200,000	210,200,000	210,200,000	210,200,000
Gas Marketing
Summary Of Significant Accounting Policies [Line Items]
Goodwill				$ 0	$ 0	$ 0	$ 0	$ 0
Natural Gas Storage Facility
Summary Of Significant Accounting Policies [Line Items]
Percentage of voting interest acquired	20.00%	100.00%	80.00%
Consideration	$ 17,000,000.0	$ 12,200,000	$ 24,800,000
Payments to acquire business			16,000,000.0
Non-interest-bearing note			10,000,000.0
Non-interest-bearing note, value			$ 8,800,000
Percentage of voting interest												20.00%
Spire Missouri
Summary Of Significant Accounting Policies [Line Items]
Average annual depreciation and amortization expense percentage				3.00%	3.00%	3.00%	3.00%	3.00%	3.00%	3.00%
Goodwill				$ 210,200,000	$ 210,200,000	$ 210,200,000	$ 210,200,000	$ 210,200,000	$ 210,200,000
Spire Missouri | ASU 2017-07
Summary Of Significant Accounting Policies [Line Items]
Reclassification of net periodic benefit costs (income) to other income (expense)									$ 17,200,000	$ 17,200,000
Spire Missouri | ASC 842 | Subsequent Event
Summary Of Significant Accounting Policies [Line Items]
Operating lease, right-of-use assets											1,900,000
Operating lease, liability											1,900,000
Spire Alabama
Summary Of Significant Accounting Policies [Line Items]
Composite depreciation rate							3.10%		3.10%	3.10%
Threshold period past due for collection				90 days
Threshold period past due for write-off of financing receivable				12 months	12 months	12 months	12 months	12 months
Spire Alabama | ASU 2017-07
Summary Of Significant Accounting Policies [Line Items]
Reclassification of net periodic benefit costs (income) to other income (expense)									$ (2,000,000.0)	$ 2,200,000
Spire Alabama | ASC 842 | Subsequent Event
Summary Of Significant Accounting Policies [Line Items]
Operating lease, right-of-use assets											10,000,000.0
Operating lease, liability											$ 10,000,000.0
Spire Alabama | Financing Receivables, Equal to Greater than 90 Days Past Due
Summary Of Significant Accounting Policies [Line Items]
Finance receivable 90 days past due				$ 400,000	$ 400,000	$ 400,000	$ 400,000	$ 400,000	$ 400,000