CONSOLIDATED BALANCE SHEETS (Parenthetical) $ in Thousands	Sep. 30, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares
Accounts receivable - trade, allowance for doubtful accounts | $	$ 1,081	$ 988
Property, plant and equipment, accumulated depreciation and amortization | $	$ 823,284	$ 789,766
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)	40,500,000	40,064,000
Common stock, outstanding shares (in shares)	40,500,000	40,064,000
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)	10	10