Stock incentive plans (Details)					12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2012 2011 Plan	Dec. 31, 2011 2011 Plan	Mar. 31, 2011 2011 Plan	Dec. 31, 2012 2001 Plan	Dec. 31, 2011 2001 Plan
Stock incentive plan
Common stock shares reserved for issuance							5,511,288
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
Outstanding stock options (in shares)	5,045,000	6,601,000	5,288,000	5,281,000	2,700,000	2,261,000		2,345,000	4,340,000
Shares of common stock granted under the plan					3,670,481