Schedule II - Valuation and Qualifying Accounts (Details) - Allowance for Doubtful Accounts: - 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 the Beginning of the Year ($)	$ 26	$ 252	$ 143
Charged to Expense ($)	10	(106)	304
Acquisitions/ Transfers and Other ($)	0	0	1
Write-offs Net of Recoveries ($)	0	(120)	(196)
Balance at the End of the Year ($)	$ 36	$ 26	$ 252