v3.8.0.1
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%