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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

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