Condensed Consolidated Balance Sheets (unaudited) (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Accounts receivable, allowances	$ 451	$ 420
Property, plant and equipment, accumulated depreciation	$ 409,053	$ 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 at par value
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)	102,245,911	101,793,829
Class B common shares at par value
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 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	$ 38,916	$ 35,902