Debt (Narrative) (Details) (USD $)	12 Months Ended			0 Months Ended		12 Months Ended	0 Months Ended		0 Months Ended		0 Months Ended		12 Months Ended			0 Months Ended	12 Months Ended		12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Mar. 24, 2011 Tower Shops January 19 2011 [Member]	Jan. 19, 2011 Tower Shops January 19 2011 [Member]	Dec. 31, 2011 Tower Shops January 19 2011 [Member]	Feb. 15, 2011 Senior Notes 4.50 Percent [Member]	Dec. 31, 2011 Senior Notes 4.50 Percent [Member]	Apr. 29, 2011 Federal Plaza [Member]	Dec. 31, 2011 Federal Plaza [Member]	Jun. 01, 2011 Tyson Station [Member]	Dec. 31, 2011 Tyson Station [Member]	Dec. 31, 2010 Revolving Credit Facility Maturing Jul 27 2011 [Member]	Dec. 31, 2011 Revolving Credit Facility Maturing Jul 27 2011 [Member]	Jul. 07, 2011 Revolving Credit Facility Maturing Jul 27 2011 [Member]	Jul. 07, 2011 Revolving Credit Facility Maturing Jul 6 2015 [Member]	Dec. 31, 2011 Revolving Credit Facility Maturing Jul 6 2015 [Member]	Dec. 31, 2010 Revolving Credit Facility Maturing Jul 6 2015 [Member]	Dec. 31, 2011 Term Loan [Member]	Dec. 01, 2011 Term Loan [Member]	Nov. 22, 2011 Term Loan [Member]	Dec. 31, 2010 Term Loan [Member]	Dec. 31, 2011 Montrose Crossing [Member]	Dec. 27, 2011 Montrose Crossing [Member]	Dec. 31, 2011 Plaza El Segundo [Member]	Dec. 30, 2011 Plaza El Segundo [Member]	Nov. 22, 2011 Interest Rate Swap [Member] agreement	Dec. 01, 2011 Interest Rate Swap [Member] Term Loan [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount					$ 41,000,000																			$ 80,000,000		$ 175,000,000
Mortgage loan fair value					42,900,000																					185,600,000
Stated Interest Rate					6.52%		4.50%	4.50%		6.75%		7.40%											4.20%	4.20%	6.33%
Percentage of prepayment premium on mortgage loan					3.00%
Repayment of mortgage loan				41,000,000					31,700,000		5,600,000
Prepayment premium on mortgage loan				1,200,000
Income from early extinguishment of debt	296,000	(2,801,000)	(2,639,000)			300,000
Unamortized debt premium						1,700,000
Repayment of senior notes	75,000,000	0	175,867,000				75,000,000
Debt Instrument, Maturity Date					Jul. 01, 2015				Jun. 01, 2011		Sep. 01, 2011
Derivative, Fixed Interest Rate																												1.72%
Total Fixed Interest Rate During Swap																				3.17%
Unsecured revolving credit facility													300,000,000		300,000,000		400,000,000
Extension Option Period																1 year
Debt Instrument, Interest Rate Terms																LIBOR plus 115 basis points
Notes Payable	295,159,000	97,881,000																	275,000,000			0
Debt Instrument, Basis Spread on Variable Rate														0.425%			1.15%		1.45%		1.45%
Debt Instrument, Inception Of No Penalty Prepayment Period																			3 years
Number of Interest Rate Derivatives Held																											2
Long-term Debt, Weighted Average Interest Rate																										6.33%
Revolving credit facility, maximum outstanding	265,000,000	82,000,000	172,500,000
Line of credit facility, weighted average amount outstanding	163,500,000	23,400,000	47,700,000
Line of credit facility weighted average interest rate before amortization of debt fees	0.96%	0.70%	1.40%
Line of credit facility, amount outstanding													77,000,000	0			0	0
Line of Credit Facility, Commitment Fee Amount													$ 500,000				$ 800,000
Debt, Weighted Average Interest Rate													0.70%
Line of Credit Facility, Expiration Date																July 6, 2015