Subsequent Events (Details) (USD $)	3 Months Ended		0 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Apr. 15, 2015	Apr. 28, 2015
Subsequent Event [Line Items]
Aggregate dividends, distributions, and dividend equivalents paid to common stockholders and common unitholders	$ 30,846,000	$ 29,561,000
Subsequent Event [Member]
Subsequent Event [Line Items]
Aggregate dividends, distributions, and dividend equivalents paid to common stockholders and common unitholders			31,900,000
San Diego Properties, San Diego, CA [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Dispositions Sales Price			95,000,000
Redmond, WA Operating Property [Member] [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Dispositions Sales Price				$ 51,200,000