| Summary of Significant Classes of Amortizable Intangible Assets |
The following tables provide a summary of our significant classes of amortizable intangible assets:
| | | 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 | |
| | | As of December 31, 2016 | | | | | | | | Accumulated | | | | | | | | Cost | | Amortization | | Net | | | | | (Dollars in thousands) | | | Customer lists and contracts | | $ | 22,599 | | $ | (20,070) | | $ | 2,529 | | | Domain and brand names | | | 19,821 | | | (12,970) | | | 6,851 | | | Favorable and assigned leases | | | 2,379 | | | (1,972) | | | 407 | | | Subscriber base and lists | | | 7,972 | | | (5,304) | | | 2,668 | | | Author relationships | | | 2,771 | | | (1,824) | | | 947 | | | Non-compete agreements | | | 2,018 | | | (1,012) | | | 1,006 | | | Other amortizable intangible assets | | | 1,336 | | | (1,336) | | | | | | | | $ | 58,896 | | $ | (44,488) | | $ | 14,408 | |
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| Amortizable Intangible Assets, Estimate Amortization Expense |
Amortization expense was approximately $4.6 million, $5.1 million and $5.3 million for the years ended December 31, 2017, 2016, and 2015, respectively. Based on the amortizable intangible assets as of December 31, 2017, we estimate amortization expense for the next five years to be as follows:
| Year ended December 31, | | Amortization Expense | | | | | (Dollars in thousands) | | | 2018 | | $ | 4,576 | | | 2019 | | | 4,006 | | | 2020 | | | 2,714 | | | 2021 | | | 1,159 | | | 2022 | | | 435 | | | Thereafter | | | 214 | | | Total | | $ | 13,104 | |
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