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Share-based compensation plans - Compensation (Details) (USD $)
In Millions, except Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2012
Employees Deferred Compensation Plan [Member]
Dec. 31, 2011
Employees Deferred Compensation Plan [Member]
Dec. 31, 2012
Non-employee Directors Deferred Compensation Plan [Member]
Dec. 31, 2011
Non-employee Directors Deferred Compensation Plan [Member]
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 year 6 months 27 days        
Unrecognized compensation cost related to previously granted nonvested shares $ 3.4        
Unrecognized compensation cost related to previously granted nonvested shares period for recognition 1 year 8 months 6 days        
Other Share-Based Compensation [Line Items]          
Common Stock Units for Deferred Compensation   421,846 947,878 64,670 55,293