Summary of Significant Accounting Policies (Computation of Dividends Paid Deduction) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Cash dividends paid	$ 318,702	$ 291,502	$ 576,832
cash dividends declared and paid in subsequent year that apply to current year	7,500	9,286	7,901
Cash Dividends Declared and Paid in Current Year That Apply to Previous Year	(9,286)	(7,901)	0
Dividends Paid Deduction	316,916	292,887	584,733
Less: Capital gain distributions	(61,531)	(63,151)	(441,577)
Total dividends paid deduction attributable to adjusted taxable income	$ 255,385	$ 229,736	$ 143,156