Supplemental Cash Flow Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental Cash Flow Information [Abstract]
Cash paid for state income taxes	$ 181,000	$ 223,000	$ 211,000
Foreign withholding taxes	$ 2,865,000	$ 14,776,000	$ 4,421,000