Debt (Details) (USD $)	0 Months Ended	3 Months Ended	6 Months Ended				0 Months Ended
	May 01, 2013	Jun. 30, 2013	Jun. 30, 2013	Jun. 30, 2012	Apr. 21, 2013	Dec. 31, 2012	Jan. 02, 2013 White Marsh Plaza [Member]	Jun. 11, 2013 Crow Canyon [Member]	May 09, 2013 Issuance of Debt [Member] Senior275PercentNotes [Member] [Member]	Jun. 09, 2013 Repayment of Debt [Member] Senior 5.40 Percent Notes [Member]
Debt Instrument [Line Items]
Senior notes and debentures		$ 1,213,333,000	$ 1,213,333,000			$ 1,076,545,000				$ 135,000,000
Long-term Debt, Gross									275,000,000
Repayments of Secured Debt							9,000,000	19,300,000
Line of Credit Facility, Maximum Borrowing Capacity		600,000,000	600,000,000		400,000,000
Basis Points Spread	90
Line of Credit Facility, Maximum Amount Outstanding During Period		76,000,000	76,000,000
Line of Credit Facility, Average Outstanding Amount		22,400,000	19,700,000
LineofCreditFacilityWeightedAverageInterestRateBeforeAmortizationOfDebt Fees		1.23%	1.29%
Line of Credit Facility, Amount Outstanding		0	0
Debt Instrument, Interest Rate, Stated Percentage									2.75%	5.40%
Early Repayment of Senior Debt										138,500,000
Proceeds from Issuance of Senior Notes, net of costs			269,343,000	0					269,300,000
Prepayment Premium On Senior Note										3,300,000
Interest Paid										$ 200,000