Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Mar. 27, 2017	Feb. 09, 2017
Revenue Recognition
Estimated amount of real estate taxes, which are the responsibility of tenants	$ 2,900,000	$ 2,700,000	$ 9,200,000	$ 7,900,000
Buena Vista, VA, Q3 2012 [Member]
Revenue Recognition
Lease termination fee receivable					$ 500,000
Buena Vista, VA, Q3 2012 [Member] | Rental income
Revenue Recognition
Gain (Loss) on Contract Termination	100,000		300,000
Belvidere, IL, Q2 2013, Six [Member]
Revenue Recognition
Lease termination fee receivable						$ 54,000
Belvidere, IL, Q2 2013, Six [Member] | Rental income
Revenue Recognition
Gain (Loss) on Contract Termination			54,000
Golden, CO, Q1 2013 [Member] | Rental income
Revenue Recognition
Gain (Loss) on Contract Termination	$ 200,000		$ 600,000