Subsequent Events (Details) (USD $)	9 Months Ended			0 Months Ended
	Sep. 30, 2012 sqft NumberofBuildings		Sep. 30, 2011	Oct. 17, 2012 Dividend Paid [Member] Subsequent Event [Member]	Oct. 05, 2012 Tribeca West, Los Angeles CA [Member] Subsequent Event [Member] NumberofBuildings sqft	Oct. 23, 2012 350 Mission St. San Francisco, CA [Member] Subsequent Event [Member]	Oct. 05, 2012 Secured Debt [Member] Tribeca West, Los Angeles CA [Member] Subsequent Event [Member]
Subsequent Event [Line Items]
Aggregate dividends, distributions, and dividend equivalents paid to common stockholders and common unitholders	$ 70,517,000		$ 58,942,000	$ 26,100,000
Purchase price	572,000,000				72,900,000	52,000,000
Number of buildings	12				1
Rentable square feet	1,532,764				151,029
Secured debt	$ 177,678,000	[1]					$ 40,700,000
Stated coupon rate							5.57%

[1] Represents the fair value of the mortgage loans assumed, which includes an aggregate unamortized premium balance of approximately $6.2 million at the dates of acquisition (see Note 5).