Summary of Significant Accounting Policies (Computation of Dividends Paid Deduction) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash dividends paid	$ 291,502	$ 576,832	$ 255,279
cash dividends declared and paid in subsequent year that apply to current year	6,500	7,901	0
Cash Dividends Declared and Paid in Current Year That Apply to Previous Year	(7,901)	0	0
Less: Return of capital	0	0	(6,717)
REIT distribution adjustment	0	0	6,717
Dividends Paid Deduction	290,101	584,733	255,279
Less: Capital gain distributions	(62,858)	(441,577)	(63,550)
Total dividends paid deduction attributable to adjusted taxable income	$ 227,243	$ 143,156	$ 191,729
REIT dividend distribution requirement percentage	90.00%