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Income taxes (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Operating Loss Carryforwards [Line Items]    
Tax benefit $ 1,000  
Deferred tax assets 205,150  
Valuation allowance 607,813 $ 839,082
Federal [Member]    
Operating Loss Carryforwards [Line Items]    
Operating loss carryforwards 1,514,243 1,510,558
Accrued and Other Current Liabilities [Member]    
Operating Loss Carryforwards [Line Items]    
Income taxes payable 12,186 $ 5,745
All U.S. and Portion of Icelandic Deferred Tax Assets [Member]    
Operating Loss Carryforwards [Line Items]    
Valuation allowance $ 607,813