Variable Interest Entities (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Consolidated Variable Interest Entities [Member] | Independence Plaza [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity, Consolidated, Carrying Amount, Assets	$ 0	$ 957,730
Variable Interest Entity, Consolidated, Carrying Amount, Liabilities	0	443,894
Variable Interest Entity Consolidated Carrying Amount Noncontrolling Interest	0	193,933
Non Consolidated Variable Interest Entities [Member] | Washington DC Office Warner Building [Member]
Variable Interest Entity [Line Items]
Carrying amount of investment in unconsolidated entity	$ 151,644	$ 8,775