Financial Instruments (Estimated Fair Values Of Financial Instruments) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Investments [Member]
Assets
Total assets, fair value	$ 4,157,077	$ 4,312,161
Financing Receivable [Member]
Assets
Total assets, fair value	5,698	5,665
Fair Value, Measurements, Recurring [Member]
Liabilities
Liabilities, fair value	3,154	77
Fair Value, Measurements, Recurring [Member] | Call Option [Member]
Liabilities
Liabilities, fair value	3,154	77
Fair Value, Measurements, Recurring [Member] | Borrowings [Member] | Unsecured debt [Member]
Liabilities
Liabilities, fair value	$ 406,350	$ 394,279