Financial Instruments (Estimated Fair Values Of Financial Instruments) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Liabilities
Liabilities, fair value	$ 1,306	$ 1,323
Call Option [Member]
Liabilities
Liabilities, fair value	505	123
Note Receivable [Member]
Assets
Total assets, fair value	5,524	5,565
Investments [Member]
Assets
Total assets, fair value	3,724,116	3,732,728
Total Return Swap [Member]
Liabilities
Liabilities, fair value	801	1,200
Borrowings [Member] | Unsecured debt [Member]
Liabilities
Liabilities, fair value	$ 375,161	$ 385,583