Summary of Significant Accounting Policies (Computation of Dividends Paid Deduction) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Cash dividends paid	$ 255,279	$ 307,169	$ 255,622
Less: Return of capital	(6,703)	0	(5,479)
REIT distribution adjustment	6,703	0	0
Dividends Paid Deduction	255,279	307,169	250,143
Less: Capital gain distributions	(63,196)	(294,901)	(95,797)
Total dividends paid deduction attributable to adjusted taxable income	$ 192,083	$ 12,268	$ 154,346