Subsequent Events (Details) (USD $)
|1 Months Ended|
Sep. 30, 2011
Northern Virginia Industrial Park II [Member]
|Proceeds from sales of business||$ 44.5|
Columbia Park Road And Dulles Business Park [Member]
|Proceeds from sales of business||$ 70.6|
Aggregate cash proceeds received from a combination of transactions in which noncurrent assets are sold, which may include the sale of a business, an investment in an affiliate (including an equity method investee), property, plant and equipment and intangible assets. Excludes sales of trading, available-for-sale, and held-to-maturity securities.