Shareholders' Equity (Details) (USD $)	9 Months Ended				27 Months Ended	30 Months Ended	60 Months Ended	9 Months Ended		66 Months Ended	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 Unit Redemption Program [Member]	Sep. 30, 2012 Unit Redemption Program [Member]	Dec. 31, 2010 Unit Redemption Program [Member]	Jun. 30, 2013 Unit Redemption Program [Member]	Sep. 30, 2013 Unit Redemption Program [Member]	Sep. 30, 2013 Dividend Reinvestment Plan [Member]	Sep. 30, 2012 Dividend Reinvestment Plan [Member]	Sep. 30, 2013 Dividend Reinvestment Plan [Member]	Sep. 30, 2013 Distributions [Member]	Sep. 30, 2012 Distributions [Member]	Sep. 30, 2013 Distributions [Member]	Sep. 30, 2013 Distributions [Member]	Sep. 30, 2012 Distributions [Member]
Shareholders' Equity (Details) [Line Items]
Unit redemption eligibility period			1 year
Redemption rate, Units owned less than 3 years			92.00%
Redemption rate, Units owned more than 3 years			100.00%
Weighted average number of Units outstanding, percentage redeemable			5.00%
Units Redeemed (in Shares)			1,100,000	1,300,000			8,500,000
Payments for Redemption of Units	$ 12,003,000	$ 13,977,000	$ 12,000,000	$ 14,000,000			$ 91,300,000
Redemption requests redeemed, description						pro-rata basis
Redemption requests redeemed, percentage					100.00%
Units Authorized (in Shares)								10,000,000		10,000,000
Stock Issued During Period, Shares, Dividend Reinvestment Plan (in Shares)								400,000	800,000	9,500,000
Stock Issued During Period, Value, Dividend Reinvestment Plan								4,300,000	8,400,000	104,200,000
Common Stock, Dividends, Per Share, Cash Paid (in Dollars per share)											$ 0.1169	$ 0.1375		$ 0.3712	$ 0.4125
Payments of Ordinary Dividends, Common Stock (in Dollars)	$ 34,305,000	$ 38,412,000									$ 10,800,000	$ 12,800,000		$ 34,300,000	$ 38,400,000
Monthly Distribution Rate (in Dollars per share)													$ 0.038958
Annual Distribution rate (in Dollars per share)													$ 0.4675