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Summary of significant accounting policies, Part 5 (Details)
shares in Millions
12 Months Ended
Dec. 31, 2017
Plan
shares
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
2006 Plan [Member] | Stock Options [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Common stock authorized for issuance under the plan (in shares) 18.9      
Common stock available for future awards (in shares) 4.9      
Assumptions used in valuing the stock options granted [Abstract]        
Expected dividend yield 0.00% 0.00% 0.00%  
Expected average life of options 4 years 3 months 18 days 4 years 3 months 18 days 4 years 3 months 18 days  
2006 Plan [Member] | Stock Options [Member] | Minimum [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Exercise price of option as percentage of fair market value at grant date, minimum 100.00%      
Assumptions used in valuing the stock options granted [Abstract]        
Expected volatility 37.00% 31.00%   34.00%
Risk-free interest rate 1.66% 0.93% 1.27%  
2006 Plan [Member] | Stock Options [Member] | Maximum [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Contractual life of awards 10 years      
Exercise period of options 10 years      
Assumptions used in valuing the stock options granted [Abstract]        
Expected volatility 40.00% 33.00%   35.00%
Risk-free interest rate 1.88% 1.22% 1.61%  
Emergent Plans [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of stock based employee compensation plans | Plan 1