Note 6 - Asset Retirement Obligations (Tables)
|12 Months Ended|
Dec. 31, 2019
|Schedule of Change in Asset Retirement Obligation [Table Text Block]||
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.
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef