Table of Contents business, expected to be completed during fiscal 2020, we abandoned the in-process research and development project totaling $4.7 million and fully amortized the intangible asset. See Note 18, "Restructuring Charges." The weighted average remaining amortization periods for existing technology, customer relationships, trade names and production know-how are approximately 2.1 years, 6.3 years, 0.1 years and 4.0 years, respectively. Amortization expense for intangible assets during fiscal 2019, 2018, and 2017 was $61.5 million, $60.0 million and $60.6 million, respectively. The change in accumulated amortization also includes $7.8 million (decrease) and $2.6 million (decrease) of foreign exchange impact for fiscal 2019 and fiscal 2018, respectively. Estimated amortization expense for the next five fiscal years and all years thereafter are as follows (in thousands): Estimated Amortization Expense 2020 $ 47,346 2021 17,403 2022 6,799 2023 4,346 2024 3,015 Thereafter 5,904 Total $ 84,813 96 COHERENT, INC. AND SUBSIDIARIES NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (Continued) 8. GOODWILL AND INTANGIBLE ASSETS (Continued)