Income Taxes (Components Of The Net Deferred Income Tax Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Components of Deferred Tax Assets and Liabilities [Abstract]
Property and Equipment	$ 92,127	$ 153,214
Long-term Debt	22,519	44,782
Unremitted earnings of foreign subsidiaries	0	179
Investments in 50% or Less Owned Companies	811	1,346
Intangible assets	1,006	1,908
Other	96	138
Total deferred tax liabilities	116,559	201,567
Share award plans	3,476	7,952
Losses on marketable securities	5,986	17,758
Deductible goodwill	312	1,611
Debt and equity issuance costs	406	7,638
Other	11,652	12,767
Total deferred tax assets	21,832	47,726
Valuation allowance	6,695	3,600
Net deferred tax assets	15,137	44,126
Net deferred tax liabilities	$ 101,422	$ 157,441