Income Taxes (Details 3) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Deferred tax assets attributable to:
Accounts receivable	$ 1,107	[1]	$ 1,345	[1]
Inventories	1,701	[1]	2,513	[1]
Employee benefit plans	25,638	[1]	19,070	[1]
Accrued liabilities	534	[1]	763	[1]
Employee incentive plans	1,305	[1]	0	[1]
Tax credit carryforwards	7,208	[1]	3,029	[1]
Available-for-sale securities	2,759	[1]	0	[1]
Other	734	[1]	6,265	[1]
Total deferred tax assets	40,986	[1]	32,985	[1]
Deferred tax liabilities attributable to:
Plant and equipment	(17,593)	[1]	(10,910)	[1]
Land and mineral reserves	(13,492)	[1]	(16,774)	[1]
Joint ventures	(1,797)	[1]	(1,541)	[1]
Intangible assets	(1,692)	[1]	(694)	[1]
Available-for-sale securities			(1,035)	[1]
Other	(550)	[1]	0	[1]
Total deferred tax liabilities	(35,124)	[1]	(30,954)	[1]
Valuation allowances	(6,750)	[1]	(3,932)	[1]
Net deferred tax assets	$ (888)	[1]	$ (1,901)	[1]

[1] Restated