Asset Purchase Agreement, Additional Percentage Share of Cost of Control

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- Definition

Asset Purchase Agreement, Operating and Maintenance Cost Reimbursement Receivable

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- Definition

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

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- Definition

Asset Purchase Agreement, Proceeds from Operating an Maintenance Cost Reimbursement

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- Definition

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

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- Definition

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

+ References

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- Definition

Supply Agreement [Abstract]

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- Definition

Surety Bonds Expiring, Amount

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- Definition

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

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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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- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 30
-Section 50
-Paragraph 10
-Subparagraph (c),(d)(2)
-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13237-110859

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 30
-Section 50
-Paragraph 10
-Subparagraph (d)(2)
-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13237-110859

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Commitments and Contingencies - Environmental Matters and Financial Assurances (Details) - USD ($) $ in Millions			3 Months Ended	12 Months Ended
	Jul. 03, 2018	Jul. 06, 2016	Mar. 31, 2019	Dec. 31, 2017	Dec. 31, 2018
APS \| Letters of Credit Expiring in 2018, 2019
Financial Assurances
Outstanding letters of credit			$ 0.2
APS \| Letters of Credit Expiring in 2019
Financial Assurances
Surety bonds expiring, amount			17.0
4C Acquisition, LLC \| Four Corners
Environmental Matters
Percentage of share of cost of control		7.00%
4C Acquisition, LLC \| Coal Supply Agreement Arbitration \| Four Corners
Four Corners Coal Supply Agreement
Proceeds from operating and maintenance cost reimbursement				$ 10.0
Reimbursement payments due to 4CA			$ 10.0		$ 20.0
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
Regional Haze Rules \| APS \| Navajo Plant
Environmental Matters
Expected environmental cost			200.0
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
Coal combustion waste \| APS \| Cholla and Four Corners
Environmental Matters
Site contingency increase in loss exposure not accrued, best estimate			5.0
Minimum \| Coal combustion waste \| APS \| Cholla
Environmental Matters
Site contingency increase in loss exposure not accrued, best estimate			$ 15.0