Share-based compensation plans (employee stock purchase plan) (Details) (Employee stock purchase plan [Member], USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Employee stock purchase plan [Member]
Employee stock purchase plan [Abstract]
Shares issued under plan (in shares)	85	109	174
Issue price for shares issued under plan, lower value (in dollars per share)	$ 20.78	$ 15.82	$ 9.80
Issue price for shares issued under plan, higher value (in dollars per share)	$ 20.01	$ 17.49	$ 13.30