Income Taxes (Deferred Taxes) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
deferred tax asset, warrant remeasurement	$ 3,974	$ 4,746
Deferred tax assets:
Net operating loss carryforward and federal credits	17,021	20,596
Post-retirement employee benefits	8,716	27,060
Employee benefits other than post-retirement	9,229	13,785
Inventory reserve	1,739	2,727
Deferred revenue	3,016	7,728
Other	4,317	4,411
Deferred tax assets	48,012	81,053
Deferred tax liabilities:
Accelerated depreciation	(129,201)	(186,015)
Partnership items	(5,858)	(8,777)
State taxes	(12,119)	(8,564)
Valuation allowance against deferred tax assets	(278)	(229)
Deferred tax liabilities	147,456	203,585
Net deferred tax (liability)	$ (99,444)	$ (122,532)