Schedule II - Valuation and Qualifying Accounts (Details) - Accounts receivable allowance [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	$ 148	$ 179	$ 465
Charged/(Credited) to Costs and Expenses, Additions and Deductions	159	(21)	(194)
Charged to Other Accounts, Additions and Deductions	0	0	0
Uncollectible Accounts Written-off	51	10	92
Balance at End of Period	$ 256	$ 148	$ 179