Variable Interest Entities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Consolidated Variable Interest Entities [Member] | Independance Plaza [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity, Consolidated, Carrying Amount, Assets	$ 942,780	$ 957,730
Variable Interest Entity, Consolidated, Carrying Amount, Liabilities	431,110	443,894
Non Consolidated Variable Interest Entities [Member] | Washington DC Office Warner Building [Member]
Variable Interest Entity [Line Items]
Carrying amount of investment in unconsolidated entity	$ 10,118	$ 8,775