Shareholders' Equity (Details) (USD $)	1 Months Ended		12 Months Ended							1 Months Ended				12 Months Ended			1 Months Ended	12 Months Ended
	Jan. 31, 2013	Jun. 30, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 6.625% Series J Preferred Stock [Member]	Dec. 31, 2012 6.500% Series K Preferred Stock [Member]	Dec. 31, 2012 6.600% Series L Preferred Stock [Member]	Dec. 31, 2012 8.375% Series O Preferred Stock [Member]	Jan. 31, 2013 Common Stock [Member]	Jun. 30, 2010 Common Stock [Member]	Jul. 31, 2011 Cumulative Preferred Stock [Member]	Feb. 28, 2011 Cumulative Preferred Stock [Member]	Dec. 31, 2012 Cumulative Preferred Stock [Member]	Dec. 31, 2010 Cumulative Preferred Stock [Member]	Dec. 31, 2011 Premier Portfolio [Member]	Jan. 31, 2013 Gross Proceeds [Member]	Dec. 31, 2012 Gross Proceeds [Member]
Total redemption value of preference shares			$ 168,272,000	$ 110,726,000	$ 118,787,000							$ 108,600,000	$ 2,100,000	$ 168,300,000	$ 118,800,000
Adjustment of ratable portion of issuance costs			5,730,000	3,796,000	10,438,000							3,600,000	163,000	5,700,000	10,400,000
Number of shares repurchased													80,000		4,500,000
Face value of shares repurchased													2,000,000		112,100,000
Limited Partner Units issued																2,100,000
Fair value of issued shares in conjunction with acquisition																28,400,000
Shares of common stock issued			22,700,000							41,400,000	26,500,000
Proceeds from issuance of common shares, net	571,900,000	298,100,000	315,295,000	0	298,004,000												590,000,000	322,200,000
Payments for Commissions			6,400,000
Preferred Stock, Selected Information [Abstract]
Shares Outstanding			2,503,000	3,176,000		396,000	598,000	796,000	712,000
Dividend Rate						6.625%	6.50%	6.60%	8.375%			7.25%	8.375%	6.95%
Liquidation Preference						$ 99,058,000	$ 149,550,000	$ 199,075,000	$ 177,955,000