| 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 September 30, 2017 | | | | | | | Accumulated | | | | | | | Cost | | Amortization | | Net | | | | | (Dollars in thousands) | | | Customer lists and contracts | | $ | 22,865 | | $ | (20,681) | | $ | 2,184 | | | Domain and brand names | | | 20,109 | | | (14,139) | | | 5,970 | | | Favorable and assigned leases | | | 2,379 | | | (2,018) | | | 361 | | | Subscriber base and lists | | | 8,797 | | | (4,402) | | | 4,395 | | | Author relationships | | | 2,771 | | | (2,171) | | | 600 | | | Non-compete agreements | | | 2,029 | | | (1,263) | | | 766 | | | Other amortizable intangible assets | | | 1,333 | | | (1,333) | | | | | | | | $ | 60,283 | | $ | (46,007) | | $ | 14,276 | |
| | | 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 $1.1 million and $1.4 million for the three month period ended September 30, 2017 and 2016, respectively, and $3.4 million and $3.7 million for the nine month period ended September 30, 2017 and 2016, respectively. Based on the amortizable intangible assets as of September 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 (Oct Dec) | | $ | 1,173 | | | 2018 | | | 4,576 | | | 2019 | | | 4,006 | | | 2020 | | | 2,714 | | | 2021 | | | 1,159 | | | Thereafter | | | 648 | | | Total | | $ | 14,276 | |
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