Amortization Of Regulatory Assets For Which No Return On Investment During Recovery Period Is Provided

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Anticipated contributions to the pension plans for fiscal year 2012 for non-qualified plans.

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Anticipated contributions to the pension plans for the next fiscal year for qualified trusts.

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Anticipated contribution to the postretirement plans for the next fiscal year for qualified trusts.

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Anticipated payment to the postretirement plans participants for the next fiscal year for non-qualified trusts.

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Reflects the upper limit on age eligibility, post early retirement, when medical insurance is available to retiree.

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Gains and losses not yet includible in pension/postretirement benefit cost are amortized only to the extent that such gain or loss exceeds 10% of the greater of the benefit obligation or the market-related value of plan assets

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No definition available.

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Disclosure under Missouri Public Service Commission Order, the return on plan assets if based on the market-related value of plan assets implemented prospectively over a four-year period.

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Number of plans.

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The amount of annual recovery of postretirement benefit costs allowed in rates by the Missouri Public Service Commission. The differences between these amounts and those calculated pursuant to GAAP are deferred as regulatory assets or liabilities.

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No definition available.

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The amount of annual recovery of net pension costs allowed in rates for qualified pension plans by the Missouri Public Service Commission. The differences between these amounts and those calculated pursuant to GAAP are deferred as regulatory assets or liabilities.

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Weighted average rate increase of compensation, used to determine benefit obligation of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan.

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Amount of (increase) decrease to benefit obligation of defined benefit plan from remeasurement due to settlement. Excludes decrease to benefit obligation for settlement payment.

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Amount of contribution received by defined benefit plan from employer which increases plan assets.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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Amount of cost for defined contribution plan.

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The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.

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Remaining recovery period of regulatory assets for which no return on investment during recovery period is provided, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Pension Plans and Other Postretirement Benefits - Additional Information (Details) $ in Millions	12 Months Ended
	Sep. 30, 2019 USD ($) plan	Sep. 30, 2018 USD ($) plan	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Apr. 19, 2019 USD ($)	Jun. 30, 2018	Apr. 19, 2018 USD ($)	Jan. 01, 2011 USD ($)	Feb. 20, 2010 USD ($)
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Implementation period used to calculate return on plan assets	4 years
Gains or losses includible in cost amortized to extent exceeds benefit obligations or market-related value of plan assets, percentage	10.00%
Regulatory assets	$ 846.2	$ 742.6
Remaining recovery period for regulatory assets for which no return on investment during recovery period is provided	15 years
Medical insurance available until age is reached after early retirement		65 years
Recovery in rates for postretirement benefit								$ 9.5
401(K) plans, cost of defined contributions	$ 13.8	$ 12.5	$ 11.5
Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory assets	536.9	471.8
401(K) plans, cost of defined contributions	10.0	9.0	8.4
Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory assets	265.1	227.7
401(K) plans, cost of defined contributions	$ 3.1	$ 3.0	$ 2.7
Spire Missouri East
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Recovery in rates for the qualified pension plans base allowance								15.5
Spire Missouri East \| Pension Cost \| MoPSC \| Infrastructure System Replacement Surcharge
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory assets							$ 29.0	$ 15.5
Spire Missouri East \| Pension and Other Postretirement Benefits
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Remaining recovery period for regulatory assets for which no return on investment during recovery period is provided	8 years
Amortization of assets	$ 173.0
Spire Missouri West
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Recovery in rates for the qualified pension plans base allowance									$ 9.9
Spire Missouri West \| Pension Cost \| MoPSC \| Infrastructure System Replacement Surcharge
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory assets							$ 5.5		$ 9.9
Spire Missouri West \| Pension and Other Postretirement Benefits
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Amortization of assets	$ 26.2
Pension Plans
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rates	3.00%	3.00%
Settlement benefits paid		$ 82.8		$ 62.2
Employer contributions	$ 27.8	58.9
Pension Plans \| Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rates				3.70%		4.20%
Settlement benefits paid		$ 65.5		$ 43.5
Number Of Plans \| plan		1
Anticipated Contributions to pension plans for qualified trust	29.0
Anticipated contributions to pension plans for non-qualified plans	0.6
Employer contributions	$ 27.1	$ 36.9
Pension Plans \| Spire Alabama
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rates		4.20%		3.65%
Settlement benefits paid		$ 17.3		$ 18.7
Number Of Plans \| plan	0	2
Anticipated Contributions to pension plans for qualified trust	$ 13.8
Employer contributions	0.5	$ 20.1
Other Pension Plan \| Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rates				3.75%		4.15%
Postretirement Benefit Costs \| Spire Missouri East \| MoPSC \| Infrastructure System Replacement Surcharge
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory assets					$ 8.6
Other Postretirement Benefits Plan
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contributions	0.6	7.6
Anticipated contributions to the postretirement plans for qualified trusts	2.0
Anticipated payment directly to participants in postretirement plan	0.5
Other Postretirement Benefits Plan \| Spire Missouri
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contributions	$ 0.6	$ 7.6
Other Postretirement Benefits Plan \| Spire Missouri East
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rates	3.00%	3.00%