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Employee plans - Fair Value Assumptions on Stock Option (Details) - $ / shares
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Retirement Benefits [Abstract]    
Weighted-average fair value of grant (in dollars per share) $ 4.53 $ 8.92
Risk-free interest rate 1.29% 1.28%
Expected volatility 51.30% 45.46%
Expected life of options in years 4 years 10 months 25 days 4 years 11 months 1 day
Expected dividend yield 0.00% 0.00%