Income Taxes (Deferred Taxes) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Deferred tax assets:
Net operating loss carryforward and federal credits	$ 30,981	$ 35,902
Post-retirement employee benefits	36,589	31,067
Employee benefits other than post-retirement	13,773	16,489
Inventory reserve	2,924	2,930
Deferred revenue	8,650	9,154
Other	2,344	1,810
Deferred tax assets	95,261	97,352
Deferred tax liabilities:
Accelerated depreciation	(175,572)	(164,858)
Partnership items	(9,489)	(9,493)
State taxes	(6,830)	(5,995)
Valuation allowance against deferred tax assets	(229)	(229)
Deferred tax liabilities	192,120	180,575
Net deferred tax (liability)	$ (96,859)	$ (83,223)