CONSOLIDATED BALANCE SHEETS (Parenthetical) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) vote $ / shares shares	Dec. 31, 2017 USD ($) vote $ / shares shares
Accounts receivable - trade, allowance for doubtful accounts | $	$ 743	$ 809
Property, plant and equipment, accumulated depreciation and amortization | $	$ 828,434	$ 811,474
Class A Common Stock
Common stock, par value (in dollars 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)	41,700,000	41,298,000
Common stock, outstanding shares (in shares)	41,700,000	41,298,000
Class B Common Stock
Common stock, par value (in dollars 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
Votes per share held | vote	10	10