Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Accounts receivable, allowances	$ 472	$ 602
Property, plant and equipment, accumulated depreciation	$ 300,899	$ 269,024
Preferred shares, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred shares, shares authorized	30,000,000	30,000,000
Preferred shares, shares issued
Class A common shares at par value
Common shares, par value (in dollars per share)	$ 0.01	$ 0.01
Common shares, shares authorized	240,000,000	240,000,000
Common shares, shares Issued	102,897,311	102,625,857
Class B common shares at par value
Common shares, par value (in dollars per share)	$ 0.01	$ 0.01
Common shares, shares authorized	120,000,000	120,000,000
Common shares, shares Issued	18,044,478	18,044,478
Reduction due to class B common shares owned by a subsidiary, shares	18,044,478	18,044,478