Condensed Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Total revenues	$ 406,717	$ 388,323	$ 818,792	$ 780,165
Costs of products sold	$ 385,114	$ 358,588	$ 748,283	$ 694,801
Type of Cost, Good or Service [Extensible List]	us-gaap:ProductMember	us-gaap:ProductMember	us-gaap:ProductMember	us-gaap:ProductMember
Gross profit (loss)	$ 21,603	$ 29,735	$ 70,509	$ 85,364
Selling, general and administrative expenses	31,561	31,545	68,624	66,422
(Gains) losses on dispositions of plant, equipment and timberlands, net	(579)	(58)	(2,133)	(26)
Operating income (loss)	(9,379)	(1,752)	4,018	18,968
Non-operating income (expense)
Interest expense	(5,814)	(4,476)	(11,009)	(8,484)
Interest income	26	45	80	158
Other, net	(257)	98	(28)	910
Total non-operating expense	(6,045)	(4,333)	(10,957)	(7,416)
Income (loss) before income taxes	(15,424)	(6,085)	(6,939)	11,552
Income tax provision (benefit)	(8,025)	(371)	(5,256)	5,663
Net income (loss)	$ (7,399)	$ (5,714)	$ (1,683)	$ 5,889
Earnings (loss) per share
Basic	$ (0.17)	$ (0.13)	$ (0.04)	$ 0.14
Diluted	(0.17)	(0.13)	(0.04)	0.13
Cash dividends declared per common share	$ 0.13	$ 0.13	$ 0.26	$ 0.26
Weighted average shares outstanding
Basic	43,770	43,604	43,735	43,593
Diluted	43,770	43,604	43,735	44,449
Product [Member]
Total revenues	$ 405,773	$ 387,342	$ 816,420	$ 778,055
Energy and Related Sales Net [Member]
Total revenues	$ 944	$ 981	$ 2,372	$ 2,110