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Fair value measurements (Details) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Debt instrument, carrying value $ 396,871 $ 396,269
Senior secured revolving credit facility    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Debt instrument, carrying value 0 0
Senior unsecured notes due October 2021    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Debt instrument, carrying value 400,000 400,000
Senior unsecured notes due October 2021 | Significant other observable inputs (Level 2)    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Debt instrument, fair value 402,000 334,100
Debt instrument, carrying value $ 402,000 $ 402,500