Fair Value of Financial Instruments (Estimated Fair Values Of Financial Instruments) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Liabilities
Liabilities	$ 293,407	$ 301,785
Investments [Member]
Assets
Investments	3,554,236	3,380,642
Options Sold [Member]
Liabilities
Liabilities	122	260
Total Return Swap [Member]
Liabilities
Liabilities	3,285	11,525
Borrowings [Members] | Secured Debt [Member]
Liabilities
Liabilities	140,000	140,000
Borrowings [Members] | Unsecured Debt [Member]
Liabilities
Liabilities	$ 150,000	$ 150,000