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Commitments and Contingencies - Environmental Matters and Financial Assurances (Details) $ in Millions |  |  |  |  | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | --- | ---
Jul. 03, 2018 USD ($) | Jun. 29, 2018 USD ($) | Jun. 13, 2017 USD ($) | Jul. 06, 2016 guarantee | Sep. 30, 2018 USD ($) | Dec. 31, 2017 USD ($) | Mar. 12, 2018 USD ($)
Coal Supply Agreement Arbitration | Four Corners |  |  |  |  |  |  |
Four Corners Coal Supply Agreement |  |  |  |  |  |  |
Damages sought |  |  | $ 30.0 |  |  |  |
Settlement amount |  | $ 45.0 |  |  |  |  |
APS | Coal Supply Agreement Arbitration | Four Corners |  |  |  |  |  |  |
Four Corners Coal Supply Agreement |  |  |  |  |  |  |
Damages sought |  |  | $ 17.0 |  |  |  |
Settlement amount |  | $ 34.0 |  |  |  |  |
Coal advance purchase |  |  |  |  |  |  | $ 24.0
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 |  |  |  |  | $ 36.0 |  |
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 |  |  |  |  |  | $ 10.0 |
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 |  |  |  |  | $ 20.0 |  |
Payment Guarantee |  |  |  |  |  |  |
Financial Assurances |  |  |  |  |  |  |
Number of parental guarantees | guarantee |  |  |  | 5 |  |  |

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