Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Accounts receivable, allowances	$ 563	$ 472
Property, plant and equipment, accumulated depreciation	$ 330,390	$ 292,412
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	0	0
Class A common shares
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	103,604,245	102,897,311
Class B common shares
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