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Commitments and Contingencies - Environmental Matters and Financial Assurances (Details) $ in Millions |  |  | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Jun. 13, 2017 USD ($) | Jul. 06, 2016 guarantee | Mar. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Mar. 12, 2018 USD ($)
Coal Supply Agreement Arbitration | Four Corners |  |  |  |  |
Four Corners Coal Supply Agreement |  |  |  |  |
Damages sought | $ 30 |  |  |  |
Settlement amount |  |  | $ 45 |  |
Arizona Public Service Company | Coal Supply Agreement Arbitration | Four Corners |  |  |  |  |
Four Corners Coal Supply Agreement |  |  |  |  |
Damages sought | $ 17 |  |  |  |
Settlement amount |  |  | 34 |  |
Coal advance purchase |  |  |  |  | $ 24
Arizona Public Service Company | Letter of Credit Expiring in 2016 and 2017 |  |  |  |  |
Financial Assurances |  |  |  |  |
Outstanding letters of credit |  |  | 5 |  |
Arizona Public Service Company | Letters of Credit Expiring in 2017 |  |  |  |  |
Financial Assurances |  |  |  |  |
Surety bonds expiring, amount |  |  | $ 36 |  |
4C Acquisition, LLC | Four Corners Units 4 and 5 |  |  |  |  |
Environmental Matters |  |  |  |  |
Percentage of share of cost of control |  |  | 7.00% |  |
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 |
Reimbursement payments due to 4CA |  |  |  | $ 20 |
NTEC | Four Corners |  |  |  |  |
Four Corners Coal Supply Agreement |  |  |  |  |
Option to purchase ownership interest (as a percent) |  | 7.00% |  |  |
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 | Arizona Public Service Company | Four Corners Units 4 and 5 |  |  |  |  |
Environmental Matters |  |  |  |  |
Percentage of share of cost of control |  |  | 63.00% |  |
Expected environmental cost |  |  | $ 400 |  |
Regional Haze Rules | Arizona Public Service Company | 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 | Arizona Public Service Company | Four Corners | Four Corners Units 4 and 5 |  |  |  |  |
Environmental Matters |  |  |  |  |
Site contingency increase in loss exposure not accrued, best estimate |  |  | $ 45 |  |
Regional Haze Rules | Arizona Public Service Company | Navajo Plant |  |  |  |  |
Environmental Matters |  |  |  |  |
Expected environmental cost |  |  | 200 |  |
Coal combustion waste | Arizona Public Service Company | Four Corners |  |  |  |  |
Environmental Matters |  |  |  |  |
Site contingency increase in loss exposure not accrued, best estimate |  |  | 22 |  |
Coal combustion waste | Arizona Public Service Company | Navajo Plant |  |  |  |  |
Environmental Matters |  |  |  |  |
Site contingency increase in loss exposure not accrued, best estimate |  |  | 1 |  |
Minimum | Coal combustion waste | Arizona Public Service Company | Cholla |  |  |  |  |
Environmental Matters |  |  |  |  |
Site contingency increase in loss exposure not accrued, best estimate |  |  | $ 20 |  |
Payment Guarantee |  |  |  |  |
Financial Assurances |  |  |  |  |
Number of parental guarantees | guarantee |  | 4 |  |  |

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