Share-based compensation plans - Compensation (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 years	Dec. 31, 2010
Share-based compensation plans [Abstract]
Unrecognized compensation cost related to previously granted stock options	$ 0.9
Unrecognized compensation cost related to previously granted stock options period for recognition	1.5
Other Share-Based Compensation [Line Items]
Common Stock Units for Deferred Compensation	55,293	50,619
Employees Deferred Compensation Plan [Member]
Other Share-Based Compensation [Line Items]
Common Stock Units for Deferred Compensation	947,878	869,745