Condensed Consolidating Financial Statements - Condensed Consolidating Statement of Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Condensed Income Statements, Captions [Line Items]
Total revenues	$ 406,717	$ 388,323	$ 818,792	$ 780,165
Costs of products sold	385,114	358,588	748,283	694,801
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
Other 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
Other comprehensive income (loss)	(25,653)	26,332	(11,633)	33,525
Comprehensive income (loss)	(33,052)	20,618	(13,316)	39,414
Adjustments/ Eliminations [Member]
Condensed Income Statements, Captions [Line Items]
Total revenues	(25,157)	(21,788)	(50,097)	(42,266)
Costs of products sold	(25,157)	(21,788)	(50,097)	(42,266)
Other non-operating income (expense)
Interest expense	1,606	1,349	2,976	2,619
Interest income	(1,606)	(1,349)	(2,976)	(2,619)
Equity in earnings of subsidiaries	(27,678)	(38,050)	(71,166)	(65,519)
Total non-operating expense	(27,678)	(38,050)	(71,166)	(65,519)
Income (loss) before income taxes	(27,678)	(38,050)	(71,166)	(65,519)
Net income (loss)	(27,678)	(38,050)	(71,166)	(65,519)
Other comprehensive income (loss)	56,736	(47,335)	33,665	(57,451)
Comprehensive income (loss)	29,058	(85,385)	(37,501)	(122,970)
Product [Member]
Condensed Income Statements, Captions [Line Items]
Total revenues	405,773	387,342	816,420	778,055
Product [Member] | Adjustments/ Eliminations [Member]
Condensed Income Statements, Captions [Line Items]
Total revenues	(25,157)	(21,788)	(50,097)	(42,266)
Energy and Related Sales Net [Member]
Condensed Income Statements, Captions [Line Items]
Total revenues	944	981	2,372	2,110
Parent Company [Member]
Condensed Income Statements, Captions [Line Items]
Total revenues	190,976	192,351	391,843	399,251
Costs of products sold	203,339	196,145	394,504	381,391
Gross profit (loss)	(12,363)	(3,794)	(2,661)	17,860
Selling, general and administrative expenses	16,971	16,421	37,408	36,583
(Gains) losses on dispositions of plant, equipment and timberlands, net	(22)	16	(460)	48
Operating income (loss)	(29,312)	(20,231)	(39,609)	(18,771)
Other non-operating income (expense)
Interest expense	(6,389)	(5,182)	(12,071)	(9,843)
Interest income	133	142	279	291
Equity in earnings of subsidiaries	14,971	18,801	36,204	32,418
Other, net	1,532	781	(881)	2,365
Total non-operating expense	10,247	14,542	23,531	25,231
Income (loss) before income taxes	(19,065)	(5,689)	(16,078)	6,460
Income tax provision (benefit)	(11,666)	25	(14,395)	571
Net income (loss)	(7,399)	(5,714)	(1,683)	5,889
Other comprehensive income (loss)	(25,653)	26,332	(11,633)	33,525
Comprehensive income (loss)	(33,052)	20,618	(13,316)	39,414
Parent Company [Member] | Product [Member]
Condensed Income Statements, Captions [Line Items]
Total revenues	190,032	191,370	389,471	397,141
Parent Company [Member] | Energy and Related Sales Net [Member]
Condensed Income Statements, Captions [Line Items]
Total revenues	944	981	2,372	2,110
Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Total revenues	32,319	23,052	52,332	42,585
Costs of products sold	31,097	22,047	49,362	40,633
Gross profit (loss)	1,222	1,005	2,970	1,952
Selling, general and administrative expenses	(332)	236	1,673	310
(Gains) losses on dispositions of plant, equipment and timberlands, net	(565)	(74)	(1,680)	(74)
Operating income (loss)	2,119	843	2,977	1,716
Other non-operating income (expense)
Interest expense	(585)	(206)	(1,040)	(319)
Interest income	1,446	1,237	2,690	2,398
Equity in earnings of subsidiaries	12,707	19,249	34,962	33,101
Other, net	529	(2,319)	(2,734)	(4,525)
Total non-operating expense	14,097	17,961	33,878	30,655
Income (loss) before income taxes	16,216	18,804	36,855	32,371
Income tax provision (benefit)	1,267	3	673	(47)
Net income (loss)	14,949	18,801	36,182	32,418
Other comprehensive income (loss)	(28,587)	23,964	(17,551)	29,066
Comprehensive income (loss)	(13,638)	42,765	18,631	61,484
Guarantors [Member] | Product [Member]
Condensed Income Statements, Captions [Line Items]
Total revenues	32,319	23,052	52,332	42,585
Non Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Total revenues	208,579	194,708	424,714	380,595
Costs of products sold	175,835	162,184	354,514	315,043
Gross profit (loss)	32,744	32,524	70,200	65,552
Selling, general and administrative expenses	14,922	14,888	29,543	29,529
(Gains) losses on dispositions of plant, equipment and timberlands, net	8		7
Operating income (loss)	17,814	17,636	40,650	36,023
Other non-operating income (expense)
Interest expense	(446)	(437)	(874)	(941)
Interest income	53	15	87	88
Other, net	(2,318)	1,636	3,587	3,070
Total non-operating expense	(2,711)	1,214	2,800	2,217
Income (loss) before income taxes	15,103	18,850	43,450	38,240
Income tax provision (benefit)	2,374	(399)	8,466	5,139
Net income (loss)	12,729	19,249	34,984	33,101
Other comprehensive income (loss)	(28,149)	23,371	(16,114)	28,385
Comprehensive income (loss)	(15,420)	42,620	18,870	61,486
Non Guarantors [Member] | Product [Member]
Condensed Income Statements, Captions [Line Items]
Total revenues	$ 208,579	$ 194,708	$ 424,714	$ 380,595