Stock incentive plans (Details)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Stock incentive plans
Number of options to purchase shares of common stock granted (in shares)	1,351,000	1,391,000	2,381,000
Number of options to purchase shares of common stock outstanding (in shares)	4,955,000	5,045,000	6,601,000	5,288,000
RSUs
Stock incentive plans
Number of RSUs granted (in shares)	799,000
2011 Plan
Stock incentive plans
Common stock shares reserved for issuance	7,108,013
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 to purchase shares of common stock outstanding (in shares)	3,096,000	2,700,000
2011 Plan | Stock Options
Stock incentive plans
Number of options to purchase shares of common stock granted (in shares)	5,026,486
2011 Plan | RSUs
Stock incentive plans
Number of RSUs granted (in shares)	799,289
2001 Plan
Stock incentive plans
Number of options to purchase shares of common stock outstanding (in shares)	1,858,543	2,345,000
2001 Plan | Stock Options
Stock incentive plans
Number of options to purchase shares of common stock outstanding (in shares)	0