Investment in Office and Parking Properties (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2014 State Property sqft		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2014 State Property sqft	Dec. 31, 2013	Dec. 31, 2012
Real Estate Properties [Line Items]
Number of real estate properties	51																51
Number of states in which investment properties are located	8																8
Area of real estate property (in square feet)	16,500,000.0																16,500,000.0
Net loss from properties acquired	$ (42,428)		$ 485		$ 9,845		$ (10,845)		$ 8,557		$ 2,306		$ 7,619		$ 1,168		$ (42,943)	$ 19,650	$ 39,395
Total revenues	126,381	[1]	115,747	[1]	110,460	[1]	104,113	[1]	77,465	[1]	73,325	[1]	72,623	[1]	68,165	[1]	456,701	291,579	210,496
Office Building
Real Estate Properties [Line Items]
Number of real estate properties	49																49
2014 Acquisitions
Real Estate Properties [Line Items]
Net loss from properties acquired																	2,100
Total revenues																	$ 28,900

[1] The amounts presented for the three months ended March 31, 2013, June 30, 2013 and September 30, 2013 differ from the amounts previously reported in the Company's Quarterly Reports on Form 10-Q as a result of discontinued operations consisting of properties sold in 2013 or classified as held for sale as of December 31, 2013.