Real Estate (Details) (USD $)	3 Months Ended								12 Months Ended			0 Months Ended		1 Months Ended	0 Months Ended	1 Months Ended	4 Months Ended	0 Months Ended	1 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 04, 2012	Jan. 24, 2014	Jan. 24, 2012 sqft Story	Aug. 21, 2012	Aug. 21, 2012 sqft Story	Dec. 31, 2012	Dec. 19, 2012	Dec. 19, 2012 sqft
Real Estate Properties [Line Items]
Total revenue of specific real estate properties									$ 13,922,000	$ 12,300,000	$ 10,217,000
Operating expenses of specific real estate properties									176,331,000	173,956,000	163,560,000
Operating income (loss)	22,526,000	23,036,000	17,726,000	20,381,000	20,407,000	21,441,000	19,373,000	19,880,000	83,669,000	81,101,000	71,906,000
Net income	10,046,000	9,090,000	5,351,000	6,658,000	6,907,000	6,258,000	4,564,000	4,865,000	31,145,000	22,594,000	51,601,000
160 King Street
Real Estate Properties [Line Items]
Sales price of a specific real estate property												93,800,000
One Beach Street
Real Estate Properties [Line Items]
Date of acquisition													Jan. 24, 2012
Rentable square feet of a specific real estate property														97,000
Number of stories in a real estate property														3
Purchase price of a specific real estate property														36,500,000
Closing costs for a specific real estate property														20,000
Business acquisition, identifiable intangible assets and liabilities, weighted average life														7 years
Total revenue of specific real estate properties											3,900,000
Operating expenses of specific real estate properties											1,000,000
Operating income (loss)											2,900,000
Net income											600,000
City Center
Real Estate Properties [Line Items]
Date of acquisition															Aug. 21, 2012
Rentable square feet of a specific real estate property															497,000	497,000
Number of stories in a real estate property															27	27
Purchase price of a specific real estate property																228,800,000
Closing costs for a specific real estate property																100,000
Business acquisition, identifiable intangible assets and liabilities, weighted average life																5 years 9 months
Approximate additional credits received to purchase price																6,900,000
Total revenue of specific real estate properties																	7,000,000
Operating expenses of specific real estate properties																	1,600,000
Operating income (loss)																	5,400,000
Geary Marketplace
Real Estate Properties [Line Items]
Date of acquisition																		Dec. 19, 2012
Rentable square feet of a specific real estate property																		35,000	35,000
Purchase price of a specific real estate property																			21,000,000
Closing costs for a specific real estate property																			$ 20,000
Business acquisition, identifiable intangible assets and liabilities, weighted average life																			19 years 9 months
Percentage of leased real estate property																		100.00%	100.00%