Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Accounts receivable, allowances	$ 271	$ 425
Property, plant and equipment, accumulated depreciation	$ 345,493	$ 338,438
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 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	102,989,249	103,979,577
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 shares owned by a subsidiary (shares)	18,044,478	18,044,478
Variable Interest Entity, Primary Beneficiary
Accounts receivable, allowances	$ 0	$ 0
Property, plant and equipment, accumulated depreciation	$ 28,525	$ 26,581