| Summary of Significant Classes of Amortizable Intangible Assets |
The following tables provide a summary of our significant classes of amortizable intangible assets: | | | As of March 31, 2018 | | | | | | | | Accumulated | | | | | | | | Cost | | Amortization | | Net | | | | | (Dollars in thousands) | | | Customer lists and contracts | | $ | 22,865 | | $ | (21,094) | | $ | 1,771 | | | Domain and brand names | | | 20,109 | | | (15,162) | | | 4,947 | | | Favorable and assigned leases | | | 2,379 | | | (2,038) | | | 341 | | | Subscriber base and lists | | | 8,797 | | | (5,307) | | | 3,490 | | | Author relationships | | | 2,771 | | | (2,302) | | | 469 | | | Non-compete agreements | | | 2,029 | | | (1,421) | | | 608 | | | Other amortizable intangible assets | | | 1,333 | | | (1,333) | | | | | | | | $ | 60,283 | | $ | (48,657) | | $ | 11,626 | | | | | As of December 31, 2017 | | | | | | | | Accumulated | | | | | | | | Cost | | Amortization | | Net | | | | | (Dollars in thousands) | | | Customer lists and contracts | | $ | 22,865 | | $ | (20,888) | | $ | 1,977 | | | Domain and brand names | | | 20,109 | | | (14,650) | | | 5,459 | | | Favorable and assigned leases | | | 2,379 | | | (2,028) | | | 351 | | | Subscriber base and lists | | | 8,797 | | | (4,701) | | | 4,096 | | | Author relationships | | | 2,771 | | | (2,237) | | | 534 | | | Non-compete agreements | | | 2,029 | | | (1,342) | | | 687 | | | Other amortizable intangible assets | | | 1,333 | | | (1,333) | | | | | | | | $ | 60,283 | | $ | (47,179) | | $ | 13,104 | |
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| Amortizable Intangible Assets, Estimate Amortization Expense |
Based on the amortizable intangible assets as of March 31, 2018, we estimate amortization expense for the next five years to be as follows: | Year Ended December 31, | | Amortization Expense | | | | | (Dollars in thousands) | | | 2018 (Apr Dec) | | $ | 4,325 | | | 2019 | | | 3,768 | | | 2020 | | | 2,343 | | | 2021 | | | 788 | | | 2022 | | | 188 | | | Thereafter | | | 214 | | | Total | | $ | 11,626 | |
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