Condensed Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data	Jun. 30, 2011	Dec. 31, 2010
Accounts receivable, allowances (in dollars)	$ 542	$ 474
Property, plant and equipment, accumulated depreciation (in dollars)	$ 307,289	$ 277,244
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	102,555,669	102,373,241
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