Fair Value of Financial Instruments (Estimated Fair Values Of Financial Instruments) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Liabilities
Liabilities, fair value	$ 1,829	$ 785
Total Return Swap [Member]
Assets
Total assets, fair value	0	667
Investments [Member]
Assets
Total assets, fair value	3,659,158	3,547,560
Total Return Swap [Member]
Liabilities
Liabilities, fair value	1,725	765
Borrowings [Member] | Secured Debt [Member]
Liabilities
Liabilities, fair value	0	140,000
Borrowings [Member] | Unsecured debt [Member]
Liabilities
Liabilities, fair value	380,059	180,000
Call Option [Member]
Liabilities
Liabilities, fair value	$ 104	$ 20