Shareholders' Equity (Details) (USD $)	1 Months Ended		12 Months Ended							1 Months Ended	12 Months Ended	1 Months Ended	12 Months Ended		1 Months Ended		12 Months Ended
	Jun. 30, 2010	Apr. 30, 2009	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 6.625% Series J Preferred Stock [Member]	Dec. 31, 2011 6.500% Series K Preferred Stock [Member]	Dec. 31, 2011 6.600% Series L Preferred Stock [Member]	Dec. 31, 2011 6.950% Series M Preferred Stock [Member]	Jul. 31, 2011 7.250% Series N Preferred Stock [Member]	Dec. 31, 2011 7.250% Series N Preferred Stock [Member]	Feb. 28, 2011 8.375% Series O Preferred Stock [Member]	Dec. 31, 2011 8.375% Series O Preferred Stock [Member]	Dec. 31, 2010 8.375% Series O Preferred Stock [Member]	Jun. 30, 2010 Common Stock [Member]	Apr. 30, 2009 Common Stock [Member]	Dec. 31, 2011 Premier Portfolio [Member]
Total redemption value of preference shares			$ 110,726,000	$ 118,787,000	$ 0					$ 108,600,000		$ 2,100,000		$ 118,800,000
Adjustment of ratable portion of issuance costs			3,796,000	10,438,000	0					3,600,000		163,000		10,400,000
Number of shares repurchased												80,000		4,500,000
Face value of shares repurchased												2,000,000		112,100,000
Issuance of shares in conjunction with acquisition																	2,100,000
Fair value of issued shares in conjunction with acquisition																	28,400,000
Shares of common stock issued															26,500,000	75,200,000
Proceeds from issuance of common shares, net	298,100,000	551,400,000	0	298,004,000	551,136,000
Preferred Stock, Selected Information [Abstract]
Shares Outstanding			3,176,000	3,618,000		396,000	598,000	796,000	673,000				712,000
Dividend Rate						6.625%	6.50%	6.60%	6.95%		7.25%		8.375%
Liquidation Preference						$ 99,058,000	$ 149,550,000	$ 199,075,000					$ 177,955,000