Fair Value Measurements (Summary of Financial Assets and Liabilities Accounted for at Fair Value) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Assets:
Investments in marketable securities	$ 201,910	$ 254,099
Derivatives	9,052	13,557
Total assets	210,962	267,656
Liabilities:
Derivatives	0	766
Level 1 [Member]
Assets:
Investments in marketable securities	11,713	5,145
Derivatives	5,597	9,026
Total assets	17,310	14,171
Level 2 [Member]
Assets:
Investments in marketable securities	190,197	248,954
Derivatives	1,023	1,491
Total assets	191,220	250,445
Liabilities:
Derivatives	0	766
Level 3 [Member]
Assets:
Derivatives	2,432	3,040
Total assets	2,432	3,040
Liabilities:
Derivatives	$ 0	$ 0