Fair Value Measurements (Financial Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets:
Investments in marketable securities	$ 135,667	$ 162,797
Derivatives	5,857	3,554
Total assets	141,524	166,351
Liabilities:
Derivatives	3,065	13,288
Level 1
Assets:
Investments in marketable securities	35,029	99,888
Derivatives	5,269	0
Total assets	40,298	99,888
Liabilities:
Derivatives		13,252
Level 2
Assets:
Investments in marketable securities	100,638	62,909
Derivatives	527	3,022
Total assets	101,165	65,931
Liabilities:
Derivatives	3,065	36
Level 3
Assets:
Investments in marketable securities	0	0
Derivatives	61	532
Total assets	61	532
Liabilities:
Derivatives	$ 0	$ 0