Summary of Significant Accounting Policies (Details) - Revenue Concentration	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Concentration, Percentage	61.00%
Base Parking Rent By Tenant [Member] | SP + [Member]
Concentration, Percentage	61.00%	58.10%
Base Parking Rent By Tenant [Member] | Premier Parking [Member]
Concentration, Percentage	15.90%	16.30%
Base Parking Rent By Tenant [Member] | Denison [Member]
Concentration, Percentage	6.40%	2.40%
Base Parking Rent By Tenant [Member] | ISOM Mgmt. [Member]
Concentration, Percentage	5.10%	4.10%
Base Parking Rent By Tenant [Member] | Interstate Parking [Member]
Concentration, Percentage	2.80%	2.90%
Base Parking Rent By Tenant [Member] | 342 N. Rampart [Member]
Concentration, Percentage	2.00%	3.10%
Base Parking Rent By Tenant [Member] | TNSH, LLC [Member]
Concentration, Percentage	1.50%	1.20%
Base Parking Rent By Tenant [Member] | BEST PARK [Member]
Concentration, Percentage	1.40%	0.30%
Base Parking Rent By Tenant [Member] | St. Louis Parking [Member]
Concentration, Percentage	1.30%	2.10%
Base Parking Rent By Tenant [Member] | Lanier [Member]
Concentration, Percentage	1.00%	2.60%
Base Parking Rent By Tenant [Member] | ABM [Member]
Concentration, Percentage	0.70%	4.30%
Base Parking Rent By Tenant [Member] | Riverside Parking [Member]
Concentration, Percentage	0.60%	1.00%
Base Parking Rent By Tenant [Member] | Denver School [Member]
Concentration, Percentage	0.20%	0.20%
Base Parking Rent By Tenant [Member] | Secure [Member]
Concentration, Percentage	0.10%	0.10%
Base Parking Rent By Tenant [Member] | Premium Parking [Member]
Concentration, Percentage		1.30%
City Concentration for Parking Base Rent [Member] | Detroit [Member]
Concentration, Percentage	24.30%	17.40%
City Concentration for Parking Base Rent [Member] | Houston [Member]
Concentration, Percentage	12.20%	12.80%
City Concentration for Parking Base Rent [Member] | Fort Worth [Member]
Concentration, Percentage	10.10%	7.80%
City Concentration for Parking Base Rent [Member] | Cincinnati [Member]
Concentration, Percentage	8.20%	8.90%
City Concentration for Parking Base Rent [Member] | Indianapolis [Member]
Concentration, Percentage	6.40%	6.20%
City Concentration for Parking Base Rent [Member] | Lubbock [Member]
Concentration, Percentage	5.10%	4.10%
City Concentration for Parking Base Rent [Member] | Cleveland [Member]
Concentration, Percentage	4.50%	6.90%
City Concentration for Parking Base Rent [Member] | Honolulu [Member]
Concentration, Percentage	4.40%	4.80%
City Concentration for Parking Base Rent [Member] | Milwaukee [Member]
Concentration, Percentage	3.70%	3.20%
City Concentration for Parking Base Rent [Member] | Nashville [Member]
Concentration, Percentage	3.70%	3.50%
City Concentration for Parking Base Rent [Member] | St Louis [Member]
Concentration, Percentage	3.60%	5.20%
City Concentration for Parking Base Rent [Member] | Minneapolis [Member]
Concentration, Percentage	2.90%	4.10%
City Concentration for Parking Base Rent [Member] | St Paul [Member]
Concentration, Percentage	2.80%	2.90%
City Concentration for Parking Base Rent [Member] | New Orleans[Member]
Concentration, Percentage	2.00%	3.10%
City Concentration for Parking Base Rent [Member] | Bridgeport [Member]
Concentration, Percentage	1.40%	2.10%
City Concentration for Parking Base Rent [Member] | Memphis [Member]
Concentration, Percentage	1.40%	1.60%
City Concentration for Parking Base Rent [Member] | San Jose [Member]
Concentration, Percentage	1.00%	2.30%
City Concentration for Parking Base Rent [Member] | Denver [Member]
Concentration, Percentage	0.70%	0.80%
City Concentration for Parking Base Rent [Member] | Louisville [Member]
Concentration, Percentage	0.60%	1.00%
City Concentration for Parking Base Rent [Member] | Clarksburg [Member]
Concentration, Percentage	0.40%	0.30%
City Concentration for Parking Base Rent [Member] | Wildwood [Member]
Concentration, Percentage	0.30%	0.40%
City Concentration for Parking Base Rent [Member] | Canton [Member]
Concentration, Percentage	0.30%	0.20%
City Concentration for Parking Base Rent [Member] | Ft. Lauderdale [Member]
Concentration, Percentage		0.40%
Real Estate Investment Concentration by City [Member] | Detroit [Member]
Concentration, Percentage	9.30%	8.80%
Real Estate Investment Concentration by City [Member] | Houston [Member]
Concentration, Percentage	8.10%	8.70%
Real Estate Investment Concentration by City [Member] | Fort Worth [Member]
Concentration, Percentage	6.70%	6.70%
Real Estate Investment Concentration by City [Member] | Cincinnati [Member]
Concentration, Percentage	6.10%	5.80%
Real Estate Investment Concentration by City [Member] | Indianapolis [Member]
Concentration, Percentage	4.50%	3.70%
Real Estate Investment Concentration by City [Member] | Lubbock [Member]
Concentration, Percentage	3.90%	4.40%
Real Estate Investment Concentration by City [Member] | Cleveland [Member]
Concentration, Percentage	4.20%	4.40%
Real Estate Investment Concentration by City [Member] | Honolulu [Member]
Concentration, Percentage	5.70%	6.20%
Real Estate Investment Concentration by City [Member] | Milwaukee [Member]
Concentration, Percentage	2.80%	2.60%
Real Estate Investment Concentration by City [Member] | Nashville [Member]
Concentration, Percentage	2.80%	2.70%
Real Estate Investment Concentration by City [Member] | St Louis [Member]
Concentration, Percentage	3.90%	3.70%
Real Estate Investment Concentration by City [Member] | Minneapolis [Member]
Concentration, Percentage	3.80%	3.80%
Real Estate Investment Concentration by City [Member] | St Paul [Member]
Concentration, Percentage	2.60%	2.60%
Real Estate Investment Concentration by City [Member] | New Orleans[Member]
Concentration, Percentage	1.20%	1.10%
Real Estate Investment Concentration by City [Member] | Bridgeport [Member]
Concentration, Percentage	1.20%	1.30%
Real Estate Investment Concentration by City [Member] | Memphis [Member]
Concentration, Percentage	1.10%	1.00%
Real Estate Investment Concentration by City [Member] | San Jose [Member]
Concentration, Percentage	1.00%	1.00%
Real Estate Investment Concentration by City [Member] | Denver [Member]
Concentration, Percentage	0.20%	0.20%
Real Estate Investment Concentration by City [Member] | Louisville [Member]
Concentration, Percentage	0.20%	0.20%
Real Estate Investment Concentration by City [Member] | Clarksburg [Member]
Concentration, Percentage	0.20%	0.40%
Real Estate Investment Concentration by City [Member] | Wildwood [Member]
Concentration, Percentage	0.20%	0.40%
Real Estate Investment Concentration by City [Member] | Canton [Member]
Concentration, Percentage		1.10%
Real Estate Investment Concentration by City [Member] | Ft. Lauderdale [Member]
Concentration, Percentage		1.10%