Financial Instruments (Estimated Fair Values Of Financial Instruments) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Investments [Member]
Assets
Total assets, fair value	$ 3,904,064	$ 3,732,728
Financing Receivable [Member]
Assets
Total assets, fair value	5,514	5,565
Fair Value, Measurements, Recurring [Member]
Liabilities
Liabilities, fair value	1,550	1,323
Fair Value, Measurements, Recurring [Member] | Call Option [Member]
Liabilities
Liabilities, fair value	589	123
Fair Value, Measurements, Recurring [Member] | Total Return Swap [Member]
Liabilities
Liabilities, fair value	961	1,200
Fair Value, Measurements, Recurring [Member] | Borrowings [Member] | Unsecured debt [Member]
Liabilities
Liabilities, fair value	$ 361,118	$ 385,583