Real Estate (Details) (USD $)	3 Months Ended								12 Months Ended			0 Months Ended	1 Months Ended	12 Months Ended	1 Months Ended	4 Months Ended	1 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 04, 2012 160 King Street	Jan. 24, 2012 One Beach Street sqft Story	Dec. 31, 2012 One Beach Street	Aug. 21, 2012 City Center sqft Story	Dec. 31, 2012 City Center	Dec. 19, 2012 Geary Marketplace sqft	Aug. 31, 2011 Valencia Corporate Center
Real Estate Properties [Line Items]
Date of acquisition									Jan. 24, 2012
Rentable square feet of a specific real estate property													97,000		497,000		35,000
Purchase price of a specific real estate property													$ 36,500,000		$ 228,800,000		$ 21,000,000
Closing costs for a specific real estate property													20,000		100,000		20,000
Business acquisition, identifiable intangible assets and liabilities, weighted average life													7 years		5 years 9 months		19 years 9 months
Number of stories in a real estate property													3		27
Approximate additional credits received to purchase price															6,900,000
Percentage of leased real estate property																	100.00%
Total revenue of specific real estate properties									12,300,000	10,217,000	8,617,000			3,900,000		7,000,000
Operating expenses of specific real estate properties									173,956,000	163,560,000	146,442,000			1,000,000		1,600,000
Operating income (loss)	20,407,000	21,441,000	19,373,000	19,880,000	20,247,000	18,206,000	16,641,000	16,812,000	81,101,000	71,906,000	56,343,000			2,900,000		5,400,000
Net income	6,907,000	6,258,000	4,564,000	4,865,000	41,821,000	4,285,000	2,624,000	2,871,000	22,594,000	51,601,000	19,324,000			600,000
Sales price of a specific real estate property												$ 93,800,000						$ 31,000,000