Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accounts receivable, allowances	$ 544	$ 420
Property, plant and equipment, accumulated depreciation	$ 417,738	$ 368,939
Preferred shares, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred shares, shares authorized (in shares)	30,000,000	30,000,000
Preferred shares, shares issued (in shares)	0	0
Class A common shares
Common shares, par value (in dollars per share)	$ 0.01	$ 0.01
Common shares, shares authorized (in shares)	240,000,000	240,000,000
Common shares, shares issued (in shares)	102,365,933	101,793,829
Class B common shares
Common shares, par value (in dollars per share)	$ 0.01	$ 0.01
Common shares, shares authorized (in shares)	120,000,000	120,000,000
Common shares, shares issued (in shares)	18,044,478	18,044,478
Reduction due to class B common shares owned by a subsidiary (in shares)	18,044,478	18,044,478
Variable interest entity, primary beneficiary
Accounts receivable, allowances	$ 0	$ 0
Property, plant and equipment, accumulated depreciation	$ 42,676	$ 35,902