Summary of Significant Accounting Policies (Computation of Dividends Paid Deduction) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Cash dividends paid	$ 576,832	$ 255,279	$ 307,169
cash dividends declared and paid in subsequent year that apply to current year	2,500	0	0
Less: Return of capital	0	(6,717)	0
REIT distribution adjustment	0	6,717	0
Dividends Paid Deduction	579,332	255,279	307,169
Less: Capital gain distributions	(441,836)	(63,550)	(294,901)
Total dividends paid deduction attributable to adjusted taxable income	$ 137,496	$ 191,729	$ 12,268
REIT dividend distribution requirement percentage	90.00%