Income Taxes (Taxability of Cash Distributions Paid on Common Shares) (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Disclosure Taxability Of Cash Distributions Paid On Common Shares [Abstract]
Ordinary income (dollars per share)	$ 2.16	$ 1.90	$ 1.64
Capital gains (dollars per share)	$ 0.48	$ 0.33	$ 0.00
Return of capital (dollars per share)	$ 0.00	$ 0.28	$ 0.82
Total (dollars per share)	$ 2.64	$ 2.51	$ 2.46