Fair Value Disclosures - Financial Assets and Liabilities Not Measured at Fair Value (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013 Estimate of Fair Value, Fair Value Disclosure [Member]	Dec. 31, 2012 Estimate of Fair Value, Fair Value Disclosure [Member]	Jun. 30, 2013 Estimate of Fair Value, Fair Value Disclosure [Member] 2445 M Street Note Receivable [Member]	Dec. 31, 2012 Estimate of Fair Value, Fair Value Disclosure [Member] 2445 M Street Note Receivable [Member]	Jun. 30, 2013 Carrying (Reported) Amount, Fair Value Disclosure [Member]	Dec. 31, 2012 Carrying (Reported) Amount, Fair Value Disclosure [Member]	Jun. 30, 2013 Carrying (Reported) Amount, Fair Value Disclosure [Member] 2445 M Street Note Receivable [Member]	Dec. 31, 2012 Carrying (Reported) Amount, Fair Value Disclosure [Member] 2445 M Street Note Receivable [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commercial paper, maturity	90
Cash and cash equivalents, Fair Value		$ 5,919	$ 19,324
Restricted Cash, Fair Value Disclosure		10,839	14,582
2445 M Street note receivable, Fair Value				7,311	6,654			6,091	6,617
Mortgage notes payable, Fair Value		337,949	374,591			312,211	342,970
Lines of credit payable, Fair Value		75,000	0
Notes payable, Fair Value		$ 865,556	$ 968,040			$ 846,450	$ 906,190