6. GOODWILL AND OTHER INTANGIBLE ASSETS December 31, 2020 December 31, 2019 Useful Life (Years) Ending Gross Amount Accumulated Amortization Ending Gross Amount Accumulated Amortization (Amounts in thousands, except years) Finite-lived intangible assets: Engineering drawings(1) 10-22 $ 90,638  $ (83,620) $ 89,490  $ (78,854) Existing customer relationships(2) 5-10 85,214  (62,796) 81,844  (53,468) Patents 9-16 27,015  (27,015) 26,132  (26,132) Other 4-40 93,923  (43,633) 92,920  (40,149) $ 296,790  $ (217,064) $ 290,386  $ (198,603) Indefinite-lived intangible assets(3) $ 90,355  $ (1,585) $ 90,607  $ (1,585) FPD FCD Total (Amounts in thousands) Balance as of December 31, 2018 $ 790,139  $ 407,501  $ 1,197,640  Currency translation and other (3,509) (1,121) (4,630) Balance as of December 31, 2019 $ 786,630  $ 406,380  $ 1,193,010  Currency translation and other 18,425  13,451  31,876  Balance as of December 31, 2020 $ 805,055  $ 419,831  $ 1,224,886  Amortization Expense (Amounts in thousands) Actual for year ended December 31, 2020 $ 13,645  Estimated for year ended December 31, 2021 14,313  Estimated for year ended December 31, 2022 12,298  Estimated for year ended December 31, 2023 9,283  Estimated for year ended December 31, 2024 4,286  Estimated for year ended December 31, 2025 2,338  Thereafter 37,208  The following schedule outlines actual amortization expense recognized during 2020 and an estimate of future amortization based upon the finite-lived intangible assets owned at December 31, 2020: Amortization expense for finite-lived intangible assets was $13.8 million in 2019 and $14.1 million in 2018. The following table provides information about our intangible assets for the years ended December 31, 2020 and 2019: The changes in the carrying amount of goodwill for the years ended December 31, 2020 and 2019 are as follows: (1) Engineering drawings represent the estimated fair value associated with specific acquired product and component schematics. (3) Accumulated amortization for indefinite-lived intangible assets relates to amounts recorded prior to the implementation date of guidance issued in ASC 350. (2) Existing customer relationships acquired prior to 2011 had a useful life of five  years . 83