| Summary of Significant Classes of Amortizable Intangible Assets |
The following tables provide details, by major category, of the significant classes of amortizable intangible assets:
| | | As of June 30, 2017 | | | | | | | Accumulated | | | | | | | Cost | | Amortization | | Net | | | | | (Dollars in thousands) | | | Customer lists and contracts | | $ | 22,577 | | $ | (20,460) | | $ | 2,117 | | | Domain and brand names | | | 19,517 | | | (13,629) | | | 5,888 | | | Favorable and assigned leases | | | 2,379 | | | (2,007) | | | 372 | | | Subscriber base and lists | | | 6,481 | | | (4,180) | | | 2,301 | | | Author relationships | | | 2,771 | | | (2,084) | | | 687 | | | Non-compete agreements | | | 2,018 | | | (1,179) | | | 839 | | | Other amortizable intangible assets | | | 1,333 | | | (1,333) | | | | | | | | $ | 57,076 | | $ | (44,872) | | $ | 12,204 | |
| | | 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 $1.2 million for each of the three month periods ended June 30, 2017 and 2016, and $2.3 million for each of the six month periods ended June 30, 2017 and 2016. Based on the amortizable intangible assets as of June 30, 2017, we estimate amortization expense for the next five years to be as follows:
| Year Ended December 31, | | Amortization Expense | | | | | (Dollars in thousands) | | | 2017 (July Dec) | | $ | 2,084 | | | 2018 | | | 3,933 | | | 2019 | | | 3,363 | | | 2020 | | | 2,072 | | | 2021 | | | 520 | | | Thereafter | | | 232 | | | Total | | $ | 12,204 | |
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