Intangible assets - Rollforward (Details) - USD ($) $ in Thousands |
1 Months Ended | 12 Months Ended | |||
|---|---|---|---|---|---|
Jan. 31, 2017 |
Nov. 30, 2016 |
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
|
| Changes in intangible assets | |||||
| Balance at beginning of year | $ 13,783 | $ 13,783 | $ 16,055 | ||
| Additions | 1,210 | ||||
| Amortization expense | (3,520) | (3,470) | |||
| Impairment loss | (12) | ||||
| Balance at end of year | 10,263 | 13,783 | $ 16,055 | ||
| Payments for asset acquisition | $ 1,150 | $ 60 | $ 0 | ||
| Caching technology intangible | |||||
| Changes in intangible assets | |||||
| Additions | $ 1,210 | ||||
| Acquisition cost of intangible asset | $ 1,250 | ||||
| Payments for asset acquisition | $ 1,150 | ||||
| Estimated useful life (in years) | 4 years | ||||
| Caching technology intangible | Cost savings and replacement cost methods | |||||
| Changes in intangible assets | |||||
| Discount rate (as a percent) | 20.00% | ||||