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Fair value measurements (Narrative) (Details) - USD ($)
Mar. 31, 2016
Dec. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Debt instrument, carrying value $ 396,252,000 $ 396,269,000
6.25% Senior Notes due October 2021    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Debt instrument, carrying value 400,000,000 400,000,000
Fair Value, Inputs, Level 2 | 6.25% Senior Notes due October 2021    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Debt instrument, fair value disclosure 346,000,000 334,100,000
Carrying Value | 6.25% Senior Notes due October 2021    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Debt instrument, carrying value 402,300,000 $ 402,500,000
Carrying Value | Fair Value, Inputs, Level 2    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Debt instrument, fair value disclosure $ 0