Shareholders' Equity (Details) (USD $)	3 Months Ended		9 Months Ended				1 Months Ended	9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011 Premier Portfolio [Member]	Sep. 30, 2011 8.375% Series O Cumulative Redeemable Preferred Shares [Member]	Jul. 31, 2011 7.25% Series N Cumulative Redeemable Preferred Shares [Member]	Sep. 30, 2011 7.25% Series N Cumulative Redeemable Preferred Shares [Member]
Number of shares repurchased						80,000
Preferred stock dividend rate, percentage						8.375%	7.25%
Face value of shares repurchased						$ 2,000,000
Total redemption value of preference shares			110,726,000	115,849,000		2,100,000		108,600,000
Adjustment of Ratable portion of issuance costs	3,633,000	5,652,000	3,796,000	10,144,000		163,000		3,600,000
Issuance of shares in conjunction with acquisition					2,100,000
Fair value of issued shares in conjunction with acquisition					$ 28,400,000