Summary of Significant Accounting Policies (Computation of Dividends Paid Deduction) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Accounting Policies [Abstract]
Cash dividends paid	$ 232,203	$ 231,446	$ 224,784
Less: Capital gain distributions	0	(62,403)	(10,828)
Less: Return of capital	(142,618)	(86,630)	(49,321)
Total dividends paid deduction attributable to adjusted taxable income	$ 89,585	$ 82,413	$ 164,635