Real Estate (Details) ft² in Thousands, $ in Thousands					1 Months Ended			3 Months Ended								4 Months Ended	12 Months Ended
	Jan. 24, 2014	Dec. 19, 2012 ft²	Dec. 04, 2012 USD ($)	Aug. 21, 2012 ft² Story	Dec. 19, 2012 USD ($) ft²	Aug. 21, 2012 USD ($) ft² Story	Jan. 24, 2012 USD ($) ft² Story	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Sep. 30, 2013 USD ($)	Jun. 30, 2013 USD ($)	Mar. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)
Real Estate Properties [Line Items]
Total revenue of specific real estate properties																	$ 13,922	$ 12,300	$ 10,217
Operating expenses of specific real estate properties																	176,331	173,956	163,560
Operating income (loss)								$ 22,526	$ 23,036	$ 17,726	$ 20,381	$ 20,407	$ 21,441	$ 19,373	$ 19,880		83,669	81,101	71,906
Net income								$ 10,046	$ 9,090	$ 5,351	$ 6,658	$ 6,907	$ 6,258	$ 4,564	$ 4,865		$ 31,145	$ 22,594	51,601
160 King Street
Real Estate Properties [Line Items]
Sales price of a specific real estate property			$ 93,800
One Beach Street
Real Estate Properties [Line Items]
Date of acquisition	Jan. 24, 2012
Rentable square feet of a specific real estate property | ft²							97
Number of stories in a real estate property | Story							3
Purchase price of a specific real estate property							$ 36,500
Closing costs for a specific real estate property							$ 20
Business acquisition, identifiable intangible assets and liabilities, weighted average life							7 years
Total revenue of specific real estate properties																			3,900
Operating expenses of specific real estate properties																			1,000
Operating income (loss)																			2,900
Net income																			$ 600
City Center
Real Estate Properties [Line Items]
Date of acquisition				Aug. 21, 2012
Rentable square feet of a specific real estate property | ft²				497		497
Number of stories in a real estate property | Story				27		27
Purchase price of a specific real estate property						$ 228,800
Closing costs for a specific real estate property						$ 100
Business acquisition, identifiable intangible assets and liabilities, weighted average life						5 years 9 months
Approximate additional credits received to purchase price						$ 6,900
Total revenue of specific real estate properties																$ 7,000
Operating expenses of specific real estate properties																1,600
Operating income (loss)																$ 5,400
Geary Marketplace
Real Estate Properties [Line Items]
Date of acquisition		Dec. 19, 2012
Rentable square feet of a specific real estate property | ft²		35			35
Purchase price of a specific real estate property					$ 21,000
Closing costs for a specific real estate property					$ 20
Business acquisition, identifiable intangible assets and liabilities, weighted average life					19 years 9 months
Percentage of leased real estate property		100.00%			100.00%