Partners' Capital (Details Textual) (USD $) In Thousands, except Share data, unless otherwise specified		6 Months Ended		3 Months Ended		6 Months Ended
	Mar. 01, 2012 Maximum [Member]	Jun. 30, 2012 Series F Preferred Stock [Member]	Jun. 30, 2012 Series F Cumulative Preferred Stock [Member]	Jun. 30, 2012 Restricted Stock Unit Awards [Member]	Jun. 30, 2011 Restricted Stock Unit Awards [Member]	Jun. 30, 2012 Restricted Stock Unit Awards [Member]	Jun. 30, 2011 Restricted Stock Unit Awards [Member]	Jun. 30, 2012 ATM [Member]	Jun. 30, 2012 ATM [Member] Common Stock [Member]	Mar. 01, 2012 ATM [Member] Common Stock [Member]
Partners' Capital (Textual) [Abstract]
Shares of common stock issuable under the At-the-Market offering	12,500,000
Aggregate gross sale proceeds of common stock under the At-the Market offering										$ 125,000
Issuance of common stock under the at-the-Market offering								1,532,598
Weighted average price of common stock issued under the At-the-Market offering								$ 12.03
Net proceeds from issuance of common stock under the At-the-Market offering								18,063
Sales agent commission									369
Restricted stock/unit awards to employees						365,137	292,339
Fair value of restricted stock/unit awards						4,327	3,248
Vesting period of restricted stock/unit awards to employees						3 years
Recognized compensation expense related to restricted stock/unit awards				1,299	1,081	2,398	1,726
Unrecognized compensation expense related to restricted stock/unit awards				$ 6,890		$ 6,890
Weighted average period during which unrecognized compensation expense is expected to be recognized						11 months 19 days
Minimum fixed coupon rate of preferred stock			2.375%
Number of years of U.S treasury rate one		30 years
Number of years of U.S treasury rate two		10 years
New coupon rate of preferred stock		5.705%	5.705%
LIBOR rate		3 month LIBOR