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Note 10 - Commitments and Contingencies (Details) - Asset Retirement Obligation Activity - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Asset Retirement Obligation Activity [Abstract]    
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