CONSOLIDATED BALANCE SHEETS (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) Vote $ / shares shares	Dec. 31, 2015 USD ($) Vote $ / shares shares
Accounts receivable - trade, allowance for doubtful accounts | $	$ 1,069	$ 988
Property, plant and equipment, accumulated depreciation and amortization | $	$ 837,122	$ 789,766
Class A Common Stock
Common stock, authorized shares	100,000,000	100,000,000
Common stock, par value (in USD per share) | $ / shares	$ 0.01	$ 0.01
Common stock, votes (in votes per share) | Vote	1
Common stock, issued shares	40,572,000	40,064,000
Common stock, outstanding shares	40,572,000	40,064,000
Class B Common Stock
Common stock, authorized shares	1,000,000	1,000,000
Common stock, par value (in USD per share) | $ / shares	$ 0.01	$ 0.01
Common stock, votes (in votes per share) | Vote	10	10
Common stock, issued shares	988,000	988,000
Common stock, outstanding shares	988,000	988,000