Schedule II Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Doubtful Accounts Receivable
Valuation Allowances and Reserves
Balance at Beginning of Period	$ 35	$ 10	$ 1
Charged to Costs and Expenses	0	45	37
Charged to Other Accounts	0	0	0
Deductions	(35)	(20)	(28)
Balance at End of Period	0	35	10
Tax Valuation Allowance
Valuation Allowances and Reserves
Balance at Beginning of Period	9,998	8,161	4,850
Charged to Costs and Expenses	3,773	1,956	3,394
Charged to Other Accounts	3,340	0	0
Deductions	0	(119)	(83)
Balance at End of Period	$ 17,111	$ 9,998	$ 8,161