Income Taxes (Details 3) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Deferred tax assets attributable to:
Accounts receivable	$ 996	$ 1,248
Inventories	1,700	2,513
Employee benefit plans	25,638	19,070
Accrued liabilities	475	703
Employee incentive plans	2,654	0
Tax credit carryforwards	7,611	7,309
Available-for-sale securities	2,759
Other	800	2,424
Total deferred tax assets	42,633	33,267
Deferred tax liabilities attributable to:
Plant and equipment	(18,136)	(10,971)
Land and mineral reserves	(897)	(959)
Joint ventures	(1,465)	(1,524)
Intangible assets	(1,692)	0
Available-for-sale securities	0	(1,022)
Other	(1,234)	(1,822)
Total deferred tax liabilities	(23,424)	(16,298)
Valuation allowances	(6,782)	(4,540)
Net deferred tax assets	$ 12,427	$ 12,429