Summary of Significant Accounting Policies - Revenue Recognition (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Revenue Recognition
Maximum period over which loan is to be considered as non-performing	60 days
Contingent rental income | $	$ 470,000	$ 457,000	$ 517,000
Maximum period over which a lease is to be considered as non-performing	60 days
Minimum
Revenue Recognition
Methods used for calculation of annual increases over the rents of the prior year	1
Specified annual increase over the prior year's rent (as a percent)	2.00%
Maximum
Revenue Recognition
Methods used for calculation of annual increases over the rents of the prior year	4
Specified annual increase over the prior year's rent (as a percent)	3.00%