Stockholders' Equity (Details Textual) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended								3 Months Ended	12 Months Ended			3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended			12 Months Ended			12 Months Ended			1 Months Ended								12 Months Ended								12 Months Ended			12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	May 31, 2011	Mar. 03, 2011	Oct. 05, 2009	Feb. 28, 2011 Maximum [Member]	May 04, 2010 Maximum [Member]	Dec. 31, 2011 Series F Preferred Stock [Member]	Dec. 31, 2011 Series F Preferred Stock [Member]	Dec. 31, 2010 Series F Preferred Stock [Member]	May 27, 2004 Series F Preferred Stock [Member]	Dec. 31, 2011 Series F Cumulative Preferred Stock [Member]	Dec. 31, 2011 Series G Cumulative Preferred Stock [Member]	Dec. 31, 2011 Series G Preferred Stock [Member]	Dec. 31, 2011 Series G Preferred Stock [Member]	Dec. 31, 2010 Series G Preferred Stock [Member]	May 27, 2004 Series G Preferred Stock [Member]	Dec. 31, 2011 Series J Preferred Stock [Member]	Dec. 31, 2010 Series J Preferred Stock [Member]	Jan. 13, 2006 Series J Preferred Stock [Member]	Dec. 31, 2011 Series K Preferred Stock [Member]	Dec. 31, 2010 Series K Preferred Stock [Member]	Aug. 21, 2006 Series K Preferred Stock [Member]	Jun. 30, 2011 Common Stock [Member]	Mar. 31, 2011 Common Stock [Member]	Oct. 31, 2009 Common Stock [Member]	Jun. 06, 2011 Common Stock [Member]	May 31, 2011 Common Stock [Member]	Mar. 04, 2011 Common Stock [Member]	Mar. 03, 2011 Common Stock [Member]	Oct. 05, 2009 Common Stock [Member]	Dec. 31, 2011 Convertible Common Stock [Member]	Dec. 31, 2010 Convertible Common Stock [Member]	Dec. 31, 2009 Convertible Common Stock [Member]	Dec. 31, 2011 ATM [Member]	Dec. 31, 2010 ATM [Member]	Dec. 31, 2011 ATM [Member] Common Stock [Member]	Dec. 31, 2010 ATM [Member] Common Stock [Member]	Feb. 28, 2011 ATM [Member] Common Stock [Member]	Dec. 31, 2010 DRIP [Member]	Dec. 31, 2009 DRIP [Member]	Dec. 31, 2011 DRIP [Member]	Dec. 31, 2010 DRIP [Member] Common Stock [Member]	Dec. 31, 2009 DRIP [Member] Common Stock [Member]
Stockholders' Equity (Textual) [Abstract]
Issue date of depositary shares												May 27, 2004						May 27, 2004			Jan. 13, 2006			Aug. 21, 2006
Depositary Shares issued									500	500	500	50,000			250	250	250	25,000	600	600	6,000,000	200	200	2,000,000
Part of each shares issued												0.01						0.01			0.0001			0.0001
Par value of each depository shares	$ 0.01	$ 0.01										$ 0.01						$ 0.01			$ 0.01			$ 0.01
Initial offering price per depository share												$ 1,000.00						$ 1,000.00			$ 25.00			$ 25.00
Redemption price per share																			$ 25.00			$ 25.00
Preferred stock dividend exception rate																			8.25%
Minimum fixed coupon rate of preferred stock													2.375%
Dividend equivalent to per depositary share																$ 72.36
Share percentage of per depositary share										6.236%				7.236%		7.236%			7.25%			7.25%
Floating rates during floating rate periods															2.50%
Number of years of U.S treasury rate one									30 years	30 years					30 years
Number of years of U.S treasury rate two									10 years	10 years					10 years
LIBOR rate									3-month LIBOR Rate	3-month LIBOR Rate					3-month LIBOR Rate
New coupon rate of preferred stock									5.365%	5.365%			5.365%
Shares of common stock																																	125,784	27,586	415,466
Operating Partnership Units resulted in reclassification	$ 0	$ 0	$ 0																														$ 1,109	$ 316	$ 7,817
Underwritten public offering																													8,400,000		8,900,000	13,635,700
Gross offering proceeds upon settlement																												102,060		101,460		71,587
Stock issuance costs related to underwriter																									1,176	890	3,042
Total expenses related to underwriting																									138	166	765
Net proceeds from underwriting																									100,746	100,404	67,780
Shares of Company Stock Issuable under an At-the-Market Offering							10,000,000	10,000,000
Aggregate gross sale proceeds																																								100,000
Issuance of company common stock																																				115,856	5,469,767				875,402	3,034,120	0
Proceeds from Common Stock Issuance ATM or DRIP																																				1,391	44,117				5,970	15,920
Sale agent commission																																						28	900
Shares awarded to directors																																												23,567	50,445
Common stock approx fair value																																									$ 128	$ 240
Stockholders' Equity (Additional Textual) [Abstract]
Previous common stock authorized for issuance		100,000,000
Common stock authorized for issuance	150,000,000	100,000,000
Common stock price per share				$ 12.15	$ 11.40	$ 5.25