Summary of Significant Accounting Policies (Computation of Dividends Paid Deduction) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Cash dividends paid	$ 355,287	$ 318,702	$ 291,502
cash dividends declared and paid in subsequent year that apply to current year	0	6,521	9,286
Cash Dividends Declared and Paid in Current Year That Apply to Previous Year	(6,521)	(9,286)	(7,901)
Dividends Paid Deduction	348,766	315,937	292,887
Less: Capital gain distributions	(90,242)	(62,513)	(63,151)
Total dividends paid deduction attributable to adjusted taxable income	$ 258,524	$ 253,424	$ 229,736