Condensed Consolidated Statements of Operations - USD ($)	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Revenues	$ 445,188,000			$ 426,460,000			$ 1,325,960,000	$ 1,295,511,000
Costs and expenses:
Cost of sales	(418,704,000)			(376,221,000)			(1,212,775,000)	(1,155,808,000)
Selling, general and administrative expenses	(28,944,000)			(26,283,000)			(85,942,000)	(85,827,000)
Gain on divested assets, net	0			22,944,000			0	22,944,000
Total operating costs and expenses	(447,648,000)			(379,560,000)			(1,298,717,000)	(1,218,691,000)
(Loss) income from operations	(2,460,000)			46,900,000			27,243,000	76,820,000
Interest expense, net	(13,077,000)			(7,547,000)			(32,477,000)	(23,290,000)
Debt retirement costs	(2,725,000)			0			(2,725,000)	0
Non-operating pension and other postretirement benefit (costs) income	(1,421,000)			(1,234,000)			(4,266,000)	(3,700,000)
(Loss) earnings before income taxes	(19,683,000)			38,119,000			(12,225,000)	49,830,000
Income tax benefit (provision)	8,710,000			(3,675,000)			4,665,000	(5,825,000)
Net (loss) earnings	$ (10,973,000)	$ (424,000)	$ 3,837,000	$ 34,444,000	$ 6,961,000	$ 2,600,000	$ (7,560,000)	$ 44,005,000
Net (loss) earnings per common share:
Basic (in dollars per share)	$ (0.66)			$ 2.09			$ (0.46)	$ 2.67
Diluted (in dollars per share)	$ (0.66)			$ 2.08			$ (0.46)	$ 2.66