Equity (Details Textual) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	1 Months Ended	6 Months Ended
	Apr. 30, 2012	Jun. 30, 2012	Jun. 30, 2011
Equity (Textual) [Abstract]
Common shares issued	4.8
Gross proceeds	$ 70,000,000	$ 300,086,000	$ 129,939,000
Common equity program, generating gross proceeds per share	$ 14.64
Ownership Interest of joint venture partner	50.00%	50.00%
Aggregate number of shares issued as per forward sale agreement		19.0
Proceeds from issuance of common shares		$ 231,200,000