CONSOLIDATED BALANCE SHEETS (Parenthetical) $ in Thousands	Jun. 30, 2017 USD ($) vote $ / shares shares	Dec. 31, 2016 USD ($) vote $ / shares shares
Accounts receivable - trade, allowance for doubtful accounts | $	$ 601	$ 1,069
Property, plant and equipment, accumulated depreciation and amortization | $	$ 783,128	$ 837,122
Class A Common Stock
Common stock, par value (in USD per share) | $ / shares	$ 0.01	$ 0.01
Common stock, authorized shares (in shares)	100,000,000	100,000,000
Common stock, issued shares (in shares)	41,042,000	40,572,000
Common stock, outstanding shares (in shares)	41,042,000	40,572,000
Class B Common Stock
Common stock, par value (in USD per share) | $ / shares	$ 0.01	$ 0.01
Common stock, authorized shares (in shares)	1,000,000	1,000,000
Common stock, issued shares (in shares)	988,000	988,000
Common stock, outstanding shares (in shares)	988,000	988,000
Common stock, votes (in votes per share) | vote	10	10