Accounting Policies and Standards Update (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Jan. 02, 2018 USD ($)	Jan. 01, 2018 T	Dec. 31, 2017 USD ($)
Accounting Standards Update 201707 [Abstract]
Cost of Goods Sold	$ 208,504	$ 205,631	$ 597,448	$ 571,600
Selling, general and administrative expenses	5,101	13,059	25,393	33,861
Operating income (loss)	717	10,257	(20,302)	2,606
Net periodic postretirement benefit income (cost), excluding service cost	20,016	1,591	23,251	4,774
Total other income (expense)	11,976	(7,933)	(4,584)	(27,819)
Accounting Standards Update 2014-09 [Abstract]
Accounts receivable	62,266		62,266				$ 50,075
Inventory, Net	69,474		69,474				72,904
Retained earnings	322,248		322,248				347,046
Accounting Standards Update 2017-07 [Member] | Scenario Previously Reported [Member]
Accounting Standards Update 201707 [Abstract]
Cost of Goods Sold		204,301		567,610
Selling, general and administrative expenses		12,798		33,077
Operating income (loss)		11,848		7,380
Net periodic postretirement benefit income (cost), excluding service cost		0		0
Total other income (expense)		(9,524)		(32,593)
Accounting Standards Update 2017-07 [Member] | Restatement Adjustment [Member]
Accounting Standards Update 201707 [Abstract]
Cost of Goods Sold		1,330		3,990
Selling, general and administrative expenses		261		784
Operating income (loss)		(1,591)		(4,774)
Net periodic postretirement benefit income (cost), excluding service cost		1,591		4,774
Total other income (expense)		$ 1,591		$ 4,774
Accounting Standards Update 2014-09 [Member]
Accounting Standards Update 2014-09 [Abstract]
Accounts receivable					$ 50,856
Inventory, Net					72,244
Retained earnings					$ 347,167
Estimated Quantity Of Tons Unrecognized | T						64,400
Accounting Standards Update 2014-09 [Member] | Difference Between Revenue Guidance In Effect Before And After Topic 606 [Member]
Accounting Standards Update 201707 [Abstract]
Cost of Goods Sold	(171)		(481)
Accounting Standards Update 2014-09 [Abstract]
Accounts receivable	262		262				781
Inventory, Net	(178)		(178)				(660)
Retained earnings	$ (71)		$ (71)				$ 121