Subsequent Events (Details) (USD $)	12 Months Ended					0 Months Ended
	Dec. 31, 2013 sqft	Dec. 31, 2012	Dec. 31, 2011	Dec. 19, 2013	Mar. 09, 2012 sqft	Feb. 11, 2014 Subsequent Event [Member]	Feb. 11, 2014 Subsequent Event [Member] Common Stock [Member]	Jan. 10, 2014 Subsequent Event [Member] Common Stock [Member]	Jan. 30, 2014 Subsequent Event [Member] JTB Center [Member] sqft	Jan. 02, 2014 Subsequent Event [Member] JTB Center [Member]	Feb. 10, 2014 Subsequent Event [Member] CalSTRS Joint Venture [Member]	Jan. 24, 2014 Subsequent Event [Member] CalSTRS Joint Venture [Member]	Jan. 17, 2014 Subsequent Event [Member] Woodbranch [Member] sqft	Jan. 31, 2014 Subsequent Event [Member] Mesa Corporate Center [Member] sqft	Feb. 10, 2014 CalSTRS Joint Venture [Member] Austin Properties [Member] Subsequent Event [Member]
Subsequent Event [Line Items]
Stock issued	31,049,976	18,951,700	0				1,325,000	10,500,000
Share price				$ 18.05			$ 18.15	$ 18.15
Proceeds from common stock issuance						$ 205,500,000
Area of real estate property (in square feet)	17,600,000				337,000				248,000				109,000	106,000
Gross purchase price		898,400,000							33,300,000		24,900,000	41,500,000	15,000,000	13,200,000
Net proceeds from the sale of business													13,900,000	12,100,000
Gain on sale of business													$ 10,000,000	$ 489,000
Occupancy rate										94.40%
Ownership interest acquired (in percent)											60.00%	17.00%
Previously acquired interest percentage															40.00%