Indebtedness (Details) (USD $)	6 Months Ended		6 Months Ended		3 Months Ended	6 Months Ended		6 Months Ended		6 Months Ended
	Jun. 30, 2014 Y	Dec. 31, 2013	Jun. 30, 2014 Line of Credit [Member] Old Credit Facility [Member]	Mar. 31, 2014 Line of Credit [Member] Old Credit Facility [Member]	Jun. 30, 2014 Line of Credit [Member] New Credit Facility [Member]	Jun. 30, 2014 Line of Credit [Member] New Credit Facility [Member]	Jun. 30, 2014 Total Debt to Total Assets Ratio [Member] Line of Credit [Member] New Credit Facility [Member]	Jun. 30, 2014 EBITDA to Fixed Charges Ratio [Member] Line of Credit [Member]	Jun. 30, 2014 Unsecured Debt to Unencumbered Asset Ratio [Member] Line of Credit [Member] New Credit Facility [Member]	Jun. 30, 2014 Unencumbered Net Operating Income to Unsecured Interest Expense [Member] Line of Credit [Member] New Credit Facility [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate					1.05%
Debt Instrument, Maturity Date			Nov. 02, 2015			Mar. 26, 2018
Length of Maturity Extension	1
Line of Credit Facility, Competitive Bid Option, Percentage						50.00%
Line of Credit Facility, Maximum Borrowing Capacity				$ 500,000,000	$ 800,000,000	$ 800,000,000
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage					0.20%
Borrowed Funds							0.60
Assets	6,731,739,000	6,775,560,000					1
Line of Credit Facility, Covenant Terms, Earnings before Interest, Taxes, Depreciation and Amortization to Fixed Charges Ratio								$ 1.50
Line of Credit Facility, Unsecured Debt to Unencumbered Asset Value, Maximum									60.00%
Line of Credit Facility, Unencumbered Net Operating Income to Unsecured Interest Expense, Minimum										175.00%