Fair Value Measurements (Summary Of Financial Assets And Liabilities Accounted For At Fair Value) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets:
Investments in marketable securities	$ 144,652	$ 155,846
Derivatives	11,262
Total assets	155,914
Liabilities:
Derivatives	786	$ 7,217
Level 1
Assets:
Investments in marketable securities	56,677
Derivatives	1,882
Total assets	58,559
Liabilities:
Derivatives	0
Level 2
Assets:
Investments in marketable securities	87,975
Derivatives	4,411
Total assets	92,386
Liabilities:
Derivatives	786
Level 3
Assets:
Investments in marketable securities	0
Derivatives	4,969
Total assets	4,969
Liabilities:
Derivatives	$ 0