Financial Instruments (Estimated Fair Values Of Financial Instruments) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Investments [Member]
Assets
Total assets, fair value	$ 4,165,002	$ 3,768,091
Financing Receivable [Member]
Assets
Total assets, fair value	5,635	5,557
Fair Value, Measurements, Recurring [Member]
Liabilities
Liabilities, fair value	2,127	4,854
Fair Value, Measurements, Recurring [Member] | Call Option [Member]
Liabilities
Liabilities, fair value	405	3
Fair Value, Measurements, Recurring [Member] | Total Return Swap [Member]
Liabilities
Liabilities, fair value	1,722	4,851
Fair Value, Measurements, Recurring [Member] | Borrowings [Member] | Unsecured debt [Member]
Liabilities
Liabilities, fair value	$ 384,401	$ 362,674