Common and Preferred Shares Common Shares (Details) (Common Stock [Member], USD $)	0 Months Ended								12 Months Ended
	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2009	Dec. 31, 2011		Dec. 31, 2010
Cash Dividends Paid [Line Items]
Common stock declared dividends per share									$ 2.80		$ 2.60
Total Cash Distribution Per Share [Member]
Cash Dividends Paid [Line Items]
Common Stock, Dividends, Per Share, Cash Paid	$ 0.70	$ 0.70	$ 0.70	$ 0.65	$ 0.65	$ 0.65	$ 0.65	$ 0.65	$ 2.75	[1]	$ 2.60	[1]
Dividend Paid Allocation By Tax Purpose, Percent									100.00%		100.00%
Taxable Ordinary Income [Member]
Cash Dividends Paid [Line Items]
Common Stock, Dividends, Per Share, Cash Paid	$ 0.5074	$ 0.5074	$ 0.5074	$ 0.4711	$ 0.2488	$ 0.2488	$ 0.2488	$ 0.2488	$ 1.9932	[1]	$ 0.9952	[1]
Dividend Paid Allocation By Tax Purpose, Percent									72.50%		38.30%
Return of Capital [Member]
Cash Dividends Paid [Line Items]
Common Stock, Dividends, Per Share, Cash Paid	$ 0.1926	$ 0.1926	$ 0.1926	$ 0.1789	$ 0.4012	$ 0.4012	$ 0.4012	$ 0.4012	$ 0.7568	[1]	$ 1.6048	[1]
Dividend Paid Allocation By Tax Purpose, Percent									27.50%		61.70%

[1] Differences between totals and details relate to rounding.