v3.8.0.1
Stock incentive plans - Plans (Details) - shares
12 Months Ended 48 Months Ended 82 Months Ended
Dec. 31, 2017
Dec. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Stock options        
Stock incentive plans        
Number of stock options granted   0    
Number of options outstanding 1,283,000 1,283,000 1,283,000 3,084,000
RSUs        
Stock incentive plans        
Number of RSUs granted (in shares) 617,000      
2011 Plan        
Stock incentive plans        
Common stock shares reserved for issuance 13,739,820 13,739,820 13,739,820  
Annual percentage increase alternative, increase in common stock reserved for issuance as a percentage of common stock outstanding 4.50%      
Annual fixed increase alternative, increase in shares of common stock reserved for issuance (in shares) 3,000,000      
Number of options outstanding 1,128,000 1,128,000 1,128,000 1,994,000
2011 Plan | Stock options        
Stock incentive plans        
Number of stock options granted     5,229,486  
2011 Plan | RSUs        
Stock incentive plans        
Number of RSUs granted (in shares)     7,713,459  
2001 Plan        
Stock incentive plans        
Number of options outstanding 155,000 155,000 155,000 1,090,000
2001 Plan | Stock options        
Stock incentive plans        
Number of options outstanding 154,699 154,699 154,699