Income Taxes (Details 3) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Asset (liability)
Post-retirement benefits other than pensions	$ 12,003	$ 11,047
Other reserves	12,804	11,476
Environmental reserves	1,958	1,932
Inventory	4,303	3,776
Pensions	27,547	21,790
Net operating loss and credit carryforwards	5,186	5,199
Miscellaneous	357	377
Subtotal	64,158	55,597
Valuation allowance	4,584	3,917
Total deferred tax assets	59,574	51,680
Depreciation	(19,388)	(21,133)
Amortization	(7,939)	(8,016)
Capitalized interest expense	(195)	(224)
Mine development	(2,832)	(782)
Derivative instruments and hedging activities	(168)	(581)
Total deferred tax liabilities	(30,522)	(30,736)
Net deferred tax asset	$ 29,052	$ 20,944