Significant Accounting Policies (Schedule of Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Significant Accounting Policies [Abstract]
Revenue from contracts with customers	$ 39,648	$ 37,330	$ 80,270	$ 73,729
Interest income	14	27	33	48
Cost recovery income
Dividend Revenue
Total revenue	$ 39,662	$ 37,357	$ 80,303	$ 73,777