the principal components of eog's rollforward of valuation allowances for deferred income tax assets for the years indicated below were as follows ( in thousands ) : .
considering the years 2016-2018 , what is the average ending balance for valuation allowances for deferred income tax assets?
( 1 ) increase in valuation allowance related to the generation of tax nols and other deferred tax assets . ( 2 ) decrease in valuation allowance associated with adjustments to certain deferred tax assets and their related allowance . ( 3 ) represents dispositions , revisions and/or foreign exchange rate variances and the effect of statutory income tax rate changes . the united kingdom operations were sold in the fourth quarter of 2018 . the argentina operations were sold in the third quarter of 2016 . as of december 31 , 2018 , eog had state income tax nols being carried forward of approximately $ 1.8 billion , which , if unused , expire between 2019 and 2037 . eog also has canadian nols of $ 183 million which can be carried forward 20 years . as described above , these nols as well as other less significant future tax benefits , have been evaluated for the likelihood of utilization , and valuation allowances have been established for the portion of these deferred income tax assets that do not meet the 201cmore likely than not 201d threshold . the balance of unrecognized tax benefits at december 31 , 2018 , was $ 29 million , resulting from the tax treatment of its research and experimental expenditures related to certain innovations in its horizontal drilling and completion projects , of which $ 12 million may potentially have an earnings impact . eog records interest and penalties related to unrecognized tax benefits to its income tax provision . currently $ 2 million of interest has been recognized in the consolidated statements of income ( loss ) and comprehensive income ( loss ) . eog does not anticipate that the amount of the unrecognized tax benefits will change materially during the next twelve months . eog and its subsidiaries file income tax returns and are subject to tax audits in the u.s . and various state , local and foreign jurisdictions . eog's earliest open tax years in its principal jurisdictions are as follows : u.s . federal ( 2016 ) , canada ( 2014 ) , trinidad ( 2013 ) and china ( 2008 ) . eog's foreign subsidiaries' undistributed earnings are not considered to be permanently reinvested outside of the u.s . accordingly , eog may be required to accrue certain u.s . federal , state , and foreign deferred income taxes on these undistributed earnings as well as on any other outside basis differences related to its investments in these subsidiaries . as of december 31 , 2018 , eog has cumulatively recorded $ 23 million of deferred foreign income taxes for withholdings on its undistributed foreign earnings . additionally , for tax years beginning in 2018 and later , eog's foreign earnings may be subject to the u.s . federal "global intangible low-taxed income" ( gilti ) inclusion . eog records any gilti tax as a period expense . 7 . employee benefit plans stock-based compensation during 2018 , eog maintained various stock-based compensation plans as discussed below . eog recognizes compensation expense on grants of stock options , sars , restricted stock and restricted stock units , performance units and grants made under the eog resources , inc . employee stock purchase plan ( espp ) . stock-based compensation expense is calculated based upon the grant date estimated fair value of the awards , net of forfeitures , based upon eog's historical employee turnover rate . compensation expense is amortized over the shorter of the vesting period or the period from date of grant until the date the employee becomes eligible to retire without company approval. .