Financial Results by Quarter Financial Results by Quarter (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Revenues	$ 143,299		$ 151,911		$ 117,540		$ 112,384		$ 109,927		$ 152,891		$ 112,370		$ 122,233		$ 525,134	$ 497,421	$ 539,447
Cost of goods sold	103,197		109,806		88,688		88,975		94,778		108,420		85,906		93,148		390,666	382,252	423,353
Selling, General and Administrative Expense	13,425		13,342		11,762		10,890		12,081		7,837		8,704		11,927		49,419	40,549	39,347
Environmental remediation charge	0		0		0		0		1,200		0		0		0		0	1,200	4,096
Asset impairment charge	107		0		0		0		0		1,180		0		0		107	1,180	0
Total costs and expenses	116,729		123,148		100,450		99,865		108,059		117,437		94,610		105,075		440,192	425,181	466,796
Operating Income	26,570		28,763		17,090		12,519		1,868		35,454		17,760		17,158		84,942	72,240	72,651
Net income (loss)	$ 13,864		$ 18,599		$ 5,080		$ 5,051		$ (1,478)		$ 25,599		$ 8,449		$ 7,696		$ 42,594	$ 40,266	$ 40,394
Basic (in dollars per share)	$ 0.34	[1]	$ 0.46	[1]	$ 0.13	[1]	$ 0.13	[1]	$ (0.04)	[1]	$ 0.64	[1]	$ 0.21	[1]	$ 0.19	[1]	$ 1.06	$ 1.00	$ 1.01
Diluted (in dollars per share)	$ 0.34	[1]	$ 0.46	[1]	$ 0.13	[1]	$ 0.13	[1]	$ (0.04)	[1]	$ 0.63	[1]	$ 0.21	[1]	$ 0.19	[1]	$ 1.05	$ 1.00	$ 1.00

[1] 1 Per-share amounts are computed independently for each of the quarters presented. Therefore, the sum of the quarterly per-share amounts may not equal the total computed for the year.