Marketable Securities and Derivative Instruments (Narratives) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Derivative [Line Items]
Proceeds from sales of, and return of investment in, marketable securities	$ 0	$ 0	$ 378,709
Other Investments [Member]
Derivative [Line Items]
Marketable Securities Gain			31,741
Proceeds from sales of, and return of investment in, marketable securities			$ 44,209