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Employee plans - Fair Value Assumptions on Stock Option (Details) - $ / shares
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Compensation and Retirement Disclosure [Abstract]      
Weighted-average fair value of grant (in dollars per share) $ 4.53 $ 8.92 $ 11.24
Risk-free interest rate 1.29% 1.28% 1.04%
Expected volatility 51.30% 45.46% 45.65%
Expected life of options in years 4 years 10 months 24 days 4 years 11 months 1 day 3 years 8 months 16 days
Expected dividend yield 0.00% 0.00% 0.00%