Debt (Narrative) (Details) (USD $)	0 Months Ended	12 Months Ended					0 Months Ended			0 Months Ended			0 Months Ended				0 Months Ended				0 Months Ended				0 Months Ended												0 Months Ended
	May 01, 2013	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 05, 2013	Apr. 21, 2013	Jan. 02, 2013 White Marsh Plaza [Member]	Dec. 31, 2013 White Marsh Plaza [Member]	Dec. 31, 2012 White Marsh Plaza [Member]	Jun. 11, 2013 Crow Canyon [Member]	Dec. 31, 2013 Crow Canyon [Member]	Dec. 31, 2012 Crow Canyon [Member]	Dec. 05, 2013 Idylwood Plaza [Member]		Dec. 31, 2013 Idylwood Plaza [Member]	Dec. 31, 2012 Idylwood Plaza [Member]	Dec. 05, 2013 Leesburg Plaza [Member]		Dec. 31, 2013 Leesburg Plaza [Member]	Dec. 31, 2012 Leesburg Plaza [Member]	Dec. 05, 2013 Loehmanns Plaza [Member]		Dec. 31, 2013 Loehmanns Plaza [Member]	Dec. 31, 2012 Loehmanns Plaza [Member]	Dec. 05, 2013 Pentagon Row [Member]		Dec. 31, 2013 Pentagon Row [Member]	Dec. 31, 2012 Pentagon Row [Member]	Dec. 31, 2013 Senior275PercentNotes [Member] [Member]	Dec. 31, 2012 Senior275PercentNotes [Member] [Member]	Dec. 31, 2013 Senior 5.40 Percent Notes [Member]	Dec. 31, 2012 Senior 5.40 Percent Notes [Member]	Dec. 31, 2013 Senior395percentnotes [Member]	Dec. 31, 2012 Senior395percentnotes [Member]	Dec. 31, 2013 Senior 5.95 Percent Notes [Member]	Dec. 31, 2012 Senior 5.95 Percent Notes [Member]	May 09, 2013 Issuance of Debt [Member] Senior275PercentNotes [Member] [Member]	Dec. 09, 2013 Issuance of Debt [Member] Senior395percentnotes [Member]	Jun. 09, 2013 Repayment of Debt [Member] Senior 5.40 Percent Notes [Member]	Dec. 27, 2013 Repayment of Debt [Member] Senior 5.95 Percent Notes [Member]
Debt Instrument [Line Items]
Repayment of mortgage loan		$ 157,300,000					$ 9,000,000			$ 19,300,000			$ 15,700,000	[1]			$ 27,300,000	[1]			$ 35,300,000	[1]			$ 50,700,000	[1]
Prepayment Premium on Mortgage Loan					4,400,000
Line of Credit Facility, Maximum Borrowing Capacity		600,000,000				400,000,000
Basis Points Spread	90
Revolving credit facility, maximum outstanding		76,000,000	186,000,000	265,000,000
Line of credit facility, weighted average amount outstanding		10,500,000	1,500,000	163,500,000
Line of credit facility weighted average interest rate before amortization of debt fees		1.30%	1.40%	1.00%
Line of Credit Facility, Commitment Fee Amount		900,000
Line of credit facility, amount outstanding		0	0
Long-term Debt, Gross								0	8,970,000		0	19,485,000			0	15,987,000			0	27,818,000			0	35,972,000			0	51,640,000	275,000,000	0	0	135,000,000	300,000,000	0	0	150,000,000	275,000,000	300,000,000
Stated Interest Rate								6.04%			5.40%				7.50%				7.50%				7.50%				7.50%		2.75%		5.40%		3.95%		5.95%		2.75%	3.95%	5.40%	5.95%
Proceeds from Issuance of Senior Long-term Debt		564,389,000	244,807,000	0																																	269,300,000	294,700,000
Senior notes and debentures		1,360,913,000	1,076,545,000																																				135,000,000	150,000,000
Early Repayment of Senior Debt																																							138,500,000	158,300,000
Prepayment Premium On Senior Note																																							3,300,000	5,000,000
Interest Paid																																							$ 200,000	$ 3,300,000

[1] The payoff included a prepayment premium totaling $4.4 million for all four mortgages which is included in "early extinguishment of debt" in 2013.