Summary of Significant Accounting Policies (Computation of Dividends Paid Deduction) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accounting Policies [Abstract]
Cash dividends paid	$ 307,170	$ 255,622	$ 251,914
Less: Return of capital	0	(5,479)	(1,938)
Dividends Paid Deduction	307,170	250,143	249,976
Less: Capital gain distributions	(294,901)	(95,797)	(109,133)
Total dividends paid deduction attributable to adjusted taxable income	$ 12,269	$ 154,346	$ 140,843