Risk Concentrations (Schedule of Foreign Revenues) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
foreign revenue [Line Items]
Sales to customers in foreign countries	$ 395,323	$ 611,917	$ 822,024
Europe [Member]
foreign revenue [Line Items]
Sales to customers in foreign countries	170,314	277,565	371,363
Asia [Member]
foreign revenue [Line Items]
Sales to customers in foreign countries	96,523	156,057	160,404
North America [Member]
foreign revenue [Line Items]
Sales to customers in foreign countries	40,315	78,445	80,322
Central and South America [Member]
foreign revenue [Line Items]
Sales to customers in foreign countries	55,323	20,496	55,493
Brokered Sales [Member]
foreign revenue [Line Items]
Sales to customers in foreign countries	$ 32,848	$ 79,354	$ 154,442