Stock option and incentive plans (Details)
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected volatility, Minimum 54.13% [1] 156.67% [1]
Expected volatility, Maximum 58.75% [1] 167.79% [1]
Expected term 6 years 6 months [2]  
Risk-free interest rate, Minimum 0.94% [3] 1.93% [3]
Risk-free interest rate, Maximum 1.60% [3] 2.19% [3]
Dividend yield 0.00% 0.00%
Minimum [Member]
    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected term   6 years [2]
Maximum [Member]
    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected term   7 years 3 months [2]
[1] The expected volatility was determined using historical volatility data for comparable companies.
[2] The expected term of the options has been estimated using the simplified method which calculates the average of the vesting period and the contractual term of the options, because the Company’s limited historical share option exercise experience does not provide a reasonable basis upon which to estimate the expected term.
[3] The risk free interest rate is based on the U.S. Treasury constant maturity nominal yield with a term approximately equal to the expected terms of the options.