Indebtedness - Credit Facility (Details) (USD $)			12 Months Ended		12 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Line of Credit [Member] Y	Dec. 31, 2012 Line of Credit [Member] Total Debt to Total Assets Ratio [Member]	Dec. 31, 2012 Line of Credit [Member] EBITDA to Fixed Charges Ratio [Member]	Dec. 31, 2012 Line of Credit [Member] Unsecured Debt to Unencumbered Asset Ratio [Member]	Dec. 31, 2012 Line of Credit [Member] Unencumbered Net Operating Income to Unsecured Interest Expense [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Date			Nov. 01, 2015
Length of Maturity Extension			1
Competitive Bid Option % of Facility Amount			50.00%
Line of Credit Facility, Amount Outstanding	$ 92,000,000	$ 139,400,000	$ 92,000,000
Line of Credit Facility, Interest Rate at Period End			1.11%
Line of Credit Facility, Commitment Fee Percentage			0.20%
Borrowed Funds for Covenant Ratio				0.60
Total Assets for Covenant Ratio	5,177,971,000	4,989,673,000		1
Earnings Before Interest, Taxes, Depreciation and Amortization for Covenant Ratio					$ 1.50
Unsecured Debt to Unencumbered Asset Value for Debt Covenant						60.00%
Unencumbered Net Operating Income to Unsecured Interest Expense Value for Debt Covenant							200.00%
Debt Instrument, Basis Spread on Variable Rate			1.075%