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Asset Purchase Agreement, Operating and Maintenance Cost Reimbursement Receivable

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Asset Purchase Agreement, Option to Purchase, Ownership Interest, Percentage

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Asset Purchase Agreement, Proceeds from Operating an Maintenance Cost Reimbursement

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Asset Purchase Agreement, Site Contingency, Increase in Loss Exposure Not Accrued, Best Estimate

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Equity Contribution Guarantees

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Represents the percentage of share of the cost of control acquired by the entity.

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Supply Agreement [Abstract]

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Surety Bonds Expiring, Amount

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The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

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For an unclassified balance sheet, amounts due from parties associated with the reporting entity as evidenced by a written promise to pay.

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Best estimate of the loss exposure for reasonably possible environmental contingencies at an individual site for which no accrual has been recorded.

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Commitments and Contingencies - Environmental Matters and Financial Assurances (Details) - USD ($) $ in Millions			9 Months Ended
	Jul. 03, 2018	Jul. 06, 2016	Sep. 30, 2020
Financial Assurances
Equity contribution guarantees			$ 39.0
APS \| Letters of Credit Expiring in 2020
Financial Assurances
Outstanding letters of credit			4.9
APS \| Surety Bonds Expiring in 2020
Financial Assurances
Surety bonds expiring, amount			16.0
4C Acquisition, LLC \| Four Corners
Environmental Matters
Percentage of share of cost of control		7.00%
Four Corners Coal Supply Agreement
Notes receivable, related parties			31.0
4C Acquisition, LLC \| Coal Supply Agreement Arbitration \| Four Corners
Four Corners Coal Supply Agreement
Reimbursement payments due to 4CA			$ 10.0
NTEC \| Four Corners
Four Corners Coal Supply Agreement
Option to purchase ownership interest (as a percent)	7.00%	7.00%
Proceeds from operating and maintenance cost reimbursement	$ 70.0
NTEC \| Coal Supply Agreement Arbitration \| Four Corners
Four Corners Coal Supply Agreement
Option to purchase ownership interest (as a percent)		7.00%
Regional Haze Rules \| APS \| Four Corners Units 4 and 5
Environmental Matters
Percentage of share of cost of control			63.00%
Expected environmental cost			$ 400.0
Regional Haze Rules \| APS \| Natural gas tolling contract obligations \| Four Corners Units 4 and 5
Environmental Matters
Additional percentage share of cost of control			7.00%
Regional Haze Rules \| APS \| Four Corners \| Four Corners Units 4 and 5
Environmental Matters
Site contingency increase in loss exposure not accrued, best estimate			$ 45.0
Coal combustion waste \| APS \| Four Corners
Environmental Matters
Site contingency increase in loss exposure not accrued, best estimate			22.0
Coal combustion waste \| APS \| Navajo Plant
Environmental Matters
Site contingency increase in loss exposure not accrued, best estimate			1.0
Minimum \| Coal combustion waste \| APS \| Cholla
Environmental Matters
Site contingency increase in loss exposure not accrued, best estimate			15.0
Minimum \| Coal combustion waste \| APS \| Cholla and Four Corners
Environmental Matters
Site contingency increase in loss exposure not accrued, best estimate			10.0
Maximum \| Coal combustion waste \| APS \| Cholla and Four Corners
Environmental Matters
Site contingency increase in loss exposure not accrued, best estimate			$ 15.0