eog utilized average prices per acre from comparable market transactions and estimated discounted cash flows as the basis for determining the fair value of unproved and proved properties , respectively , received in non-cash property exchanges . see note 10 . fair value of debt . at december 31 , 2018 and 2017 , respectively , eog had outstanding $ 6040 million and $ 6390 million aggregate principal amount of senior notes , which had estimated fair values of approximately $ 6027 million and $ 6602 million , respectively . the estimated fair value of debt was based upon quoted market prices and , where such prices were not available , other observable ( level 2 ) inputs regarding interest rates available to eog at year-end . 14 . accounting for certain long-lived assets eog reviews its proved oil and gas properties for impairment purposes by comparing the expected undiscounted future cash flows at a depreciation , depletion and amortization group level to the unamortized capitalized cost of the asset . the carrying values for assets determined to be impaired were adjusted to estimated fair value using the income approach described in the fair value measurement topic of the asc . in certain instances , eog utilizes accepted offers from third-party purchasers as the basis for determining fair value . during 2018 , proved oil and gas properties with a carrying amount of $ 139 million were written down to their fair value of $ 18 million , resulting in pretax impairment charges of $ 121 million . during 2017 , proved oil and gas properties with a carrying amount of $ 370 million were written down to their fair value of $ 146 million , resulting in pretax impairment charges of $ 224 million . impairments in 2018 , 2017 and 2016 included domestic legacy natural gas assets . amortization and impairments of unproved oil and gas property costs , including amortization of capitalized interest , were $ 173 million , $ 211 million and $ 291 million during 2018 , 2017 and 2016 , respectively . 15 . asset retirement obligations the following table presents the reconciliation of the beginning and ending aggregate carrying amounts of short-term and long-term legal obligations associated with the retirement of property , plant and equipment for the years ended december 31 , 2018 and 2017 ( in thousands ) : .
( 1 ) includes settlements related to asset sales . the current and noncurrent portions of eog's asset retirement obligations are included in current liabilities - other and other liabilities , respectively , on the consolidated balance sheets. .
2018 2017
Carrying Amount at Beginning of Period $946,848 $912,926
Liabilities Incurred 79,057 54,764
Liabilities Settled<sup>(1)</sup> (70,829) (61,871)
Accretion 36,622 34,708
Revisions (38,932) (9,818)
Foreign Currency Translations 1,611 16,139
Carrying Amount at End of Period $954,377 $946,848
Current Portion $26,214 $19,259
Noncurrent Portion $928,163 $927,589
2018 2017
carrying amount at beginning of period $ 946848 $ 912926
liabilities incurred 79057 54764
liabilities settled ( 1 ) -70829 ( 70829 ) -61871 ( 61871 )
accretion 36622 34708
revisions -38932 ( 38932 ) -9818 ( 9818 )
foreign currency translations 1611 16139
carrying amount at end of period $ 954377 $ 946848
current portion $ 26214 $ 19259
noncurrent portion $ 928163 $ 927589
what is the increase observed in the liabilities incurred during 2017 and 2018?