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Stock appreciation rights (''SARs'') (Details) - Stock Appreciation Rights (SARs) [Member] - $ / shares
Mar. 06, 2015
Jun. 01, 2014
Mar. 12, 2014
Aug. 01, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
SAR Awarded (in shares) 37,797 5,595 22,118 1,078,125
Exercise price (in dollars per share) $ 10.25 $ 13.91 $ 13.66 $ 14.00
Vesting period 3 years 3 years 3 years 5 years
Grant price (in dollars per share) $ 10.25 $ 13.91 $ 13.66 $ 14.00
Dividend yield (in hundredths) 3.90% 2.88% 2.93% 2.87%
Risk-free rate of return (in hundredths) 1.90% 2.20% 2.06% 2.15%
Expected volatility (in hundredths) 61.38% 53.60% 56.31% 54.89%
Weighted average fair value (in dollars per share) $ 2.98 $ 4.20 $ 4.17 $ 4.28
Maximum [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Average expected exercise life 5 years 5 years 5 years 6 years
Minimum [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Average expected exercise life 4 years 2 months 12 days 4 years 6 months 4 years 7 months 6 days 4 years 10 months 24 days