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Asset Retirement Obligations (Tables) | 12 Months Ended
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Dec. 31, 2017
Asset Retirement Obligation Disclosure [Abstract] |
Schedule of Asset Retirement Obligations [Table Text Block] | AROs consist of the following (in millions):
| As of December 31,
| 2017 |  | 2016
Trojan decommissioning activities | $ | 45 |  |  | $ | 44 |
Utility plant | 109 |  |  | 105 |
Non-utility property | 13 |  |  | 12 |
Asset retirement obligations | $ | 167 |  |  | $ | 161 |
Schedule of Change in Asset Retirement Obligation [Table Text Block] | The following is a summary of the changes in the Company’s AROs (in millions):
| Years Ended December 31,
| 2017 |  | 2016 |  | 2015
Balance as of beginning of year | $ | 161 |  |  | $ | 151 |  |  | $ | 116 |
Liabilities incurred | 2 |  |  | 1 |  |  | 2 |
Liabilities settled | (3 | ) |  | (3 | ) |  | (4 | )
Accretion expense | 7 |  |  | 7 |  |  | 7 |
Revisions in estimated cash flows | — |  |  | 5 |  |  | 30 |
Balance as of end of year | $ | 167 |  |  | $ | 161 |  |  | $ | 151 |

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- Definition Tabular disclosure of the carrying amount of a liability for asset retirement obligations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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- Definition Tabular disclosure of the changes in carrying amount of a liability for asset retirement obligations, for changes such as new obligations, changes in estimates of existing obligations, spending on existing obligations, property dispositions, and foreign currency translation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13201-110859 + Details
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