Investment in Unconsolidated and Consolidated Joint Ventures - Condensed Statements of Operations and Balance Sheet Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Balance Sheet Information
Assets	$ 215,203,000	$ 200,109,000
Borrowings	(114,272,000)	(97,318,000)
Equity	96,107,000	99,265,000
Investment In	30,031,000	33,803,000
Statement of Operations
Revenues	135,568,000	127,948,000	$ 127,729,000
Earnings(Loss)	6,371,000	11,782,000	10,523,000
Equity in Earnings (Loss)	4,227,000	7,098,000	6,324,000
Petro Travel Plaza Holdings, LLC
Balance Sheet Information
Assets	67,435,000	68,652,000
Borrowings	(15,279,000)	(15,275,000)
Equity	49,705,000	51,287,000
Investment In	17,422,000	18,372,000
Statement of Operations
Revenues	119,463,000	114,331,000	115,313,000
Earnings(Loss)	10,418,000	12,077,000	10,629,000
Equity in Earnings (Loss)	6,251,000	7,246,000	6,377,000
Five West Parcel, LLC
Balance Sheet Information
Assets	15,738,000	16,614,000
Borrowings	(9,711,000)	(10,251,000)
Equity	5,972,000	6,043,000
Investment In	2,802,000	2,837,000
Statement of Operations
Revenues	2,824,000	2,887,000	3,408,000
Earnings(Loss)	905,000	1,029,000	1,084,000
Equity in Earnings (Loss)	452,000	515,000	542,000
18-19 West, LLC
Balance Sheet Information
Assets	4,704,000	4,623,000
Borrowings	0	0
Equity	4,704,000	4,621,000
Investment In	1,782,000	1,741,000
Statement of Operations
Revenues	11,000	10,000	20,000
Earnings(Loss)	(97,000)	(129,000)	(108,000)
Equity in Earnings (Loss)	(48,000)	(65,000)	(54,000)
TRCC/Rock Outlet Center, LLC
Balance Sheet Information
Assets	81,610,000	86,056,000
Borrowings	(48,769,000)	(50,712,000)
Equity	32,177,000	34,523,000
Investment In	8,025,000	9,198,000
Statement of Operations
Revenues	9,615,000	9,542,000	8,988,000
Earnings(Loss)	(2,347,000)	(367,000)	(1,082,000)
Equity in Earnings (Loss)	(1,173,000)	(184,000)	(541,000)
Non-cash tenant allowance amortization	1,800,000	1,900,000	2,100,000
TRC-MRC 1, LLC
Balance Sheet Information
Assets	25,380,000	199,000
Borrowings	(19,433,000)	0
Equity	4,541,000	199,000
Investment In	0	224,000
Statement of Operations
Revenues	0	0	0
Earnings(Loss)	(3,000)	0	0
Equity in Earnings (Loss)	(2,000)	0	0
TRC-MRC 2, LLC
Balance Sheet Information
Assets	20,336,000	23,965,000
Borrowings	(21,080,000)	(21,080,000)
Equity	(992,000)	2,592,000
Investment In	0	1,431,000
Statement of Operations
Revenues	3,655,000	1,178,000	0
Earnings(Loss)	(2,505,000)	(828,000)	0
Equity in Earnings (Loss)	(1,253,000)	(414,000)	0
Non-cash amortization of purchase accounting adjustments related to in-place leases	4,000,000	1,200,000
Centennial Founders, LLC
Balance Sheet Information
Assets	89,721,000	86,099,000
Borrowings	0	0
Equity	88,862,000	85,281,000
Statement of Operations
Revenues	456,000	520,000	749,000
Earnings(Loss)	$ (144,000)	$ (246,000)	$ (140,000)