Risk Concentrations (Schedule of Foreign Revenues) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Europe [Member]
foreign revenue [Line Items]
Sales to customers in foreign countries	$ 277,565	$ 371,363	$ 674,754
Asia [Member]
foreign revenue [Line Items]
Sales to customers in foreign countries	156,057	160,404	203,193
North America [Member]
foreign revenue [Line Items]
Sales to customers in foreign countries	78,445	80,322	72,542
South America [Member]
foreign revenue [Line Items]
Sales to customers in foreign countries	20,496	55,493	57,184
Brokered Sales [Member]
foreign revenue [Line Items]
Sales to customers in foreign countries	79,354	154,442	145,438
Total [Member]
foreign revenue [Line Items]
Sales to customers in foreign countries	$ 611,917	$ 822,024	$ 1,153,111