Shareholders' Equity - Share-based payments and shareholders' rights plan (Details) - Rights Plan	12 Months Ended
	Dec. 31, 2012 item $ / shares shares
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Number of rights for each outstanding share	1
Percentage of beneficial ownership	25%
Number of business days	10 days
Exercise price | $ / shares	$ 32.00
Event Any Person Becomes Acquiring Person [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Number of times the exercise price | item	3
Common | Event Any Person Becomes Acquiring Person [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Number of rights for each outstanding share	2
Series A Preferred Stock
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Number of shares for each right	0.01
Series A Preferred Stock | Event Any Person Becomes Acquiring Person [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Number of shares for each right	0.02