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Note 10 - Commitments and Contingencies (Tables) | 12 Months Ended
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Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract] |
Schedule of Asset Retirement Obligations [Table Text Block] | The following table provides asset retirement obligation activity (in millions):
|  | For the Year Ended December 31, |
|  | 2017 |  |  | 2016 |
Asset Retirement Obligations at start of period |  | $ | 38 |  |  | $ | 43 |
Liabilities incurred in the current period |  |  | 2 |  |  |  | — |
Liabilities settled in the current period |  |  | (1 | ) |  |  | (7 | )
Accretion expense |  |  | 1 |  |  |  | 1 |
Revision in estimated cash flows |  |  | 3 |  |  |  | 1 |
Asset Retirement Obligations at end of period |  | $ | 43 |  |  | $ | 38 |

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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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