Indebtedness (Details) (USD $)	9 Months Ended			3 Months Ended	9 Months Ended		9 Months Ended		9 Months Ended	3 Months Ended
	Sep. 30, 2014 Y	Dec. 31, 2013	Mar. 31, 2014 Line of Credit [Member] Old Credit Facility [Member]	Sep. 30, 2014 Line of Credit [Member] New Credit Facility [Member]	Sep. 30, 2014 Line of Credit [Member] New Credit Facility [Member]	Sep. 30, 2014 Total Debt to Total Assets Ratio [Member] Line of Credit [Member] New Credit Facility [Member]	Sep. 30, 2014 EBITDA to Fixed Charges Ratio [Member] Line of Credit [Member]	Sep. 30, 2014 Unsecured Debt to Unencumbered Asset Ratio [Member] Line of Credit [Member] New Credit Facility [Member]	Sep. 30, 2014 Unencumbered Net Operating Income to Unsecured Interest Expense [Member] Line of Credit [Member] New Credit Facility [Member]	Sep. 30, 2014 Unsecured Notes Senior Notes [Member] 5.65% Senior Note [Member]
Credit Facility [Abstract]
Line of Credit Facility, Maximum Borrowing Capacity			$ 500,000,000	$ 800,000,000	$ 800,000,000
Length of Maturity Extension	1
Debt Instrument, Basis Spread on Variable Rate				1.05%
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage				0.20%
Line of Credit Facility, Competitive Bid Option, Percentage					50.00%
Borrowed Funds						0.60
Assets	6,660,842,000	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%
Unsecured Notes [Abstract]
Interest Rate										5.65%
Repayment of unsecured note										$ 200,000,000