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Share-based compensation Nonqualified Stock Options (Details) - Nonqualified stock options (NQSOs)
12 Months Ended
Dec. 31, 2013
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]  
Outstanding at the beginning of the period | shares 14,000
Exercised | shares (14,000)
Outstanding at the end of the period | shares 0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]  
Outstanding, at the beginning of the period (in dollars per share) | $ / shares $ 20.49
Exercised (in dollars per share) | $ / shares 20.49
Outstanding, at the end of the period (in dollars per share) | $ / shares $ 0.00