Schedule II - Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Additions Charged to Cost and Expenses	$ (3,950)	$ (13,028)	$ (3,613)
Allowance for doubtful accounts
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	23	33	42
Additions Charged to Cost and Expenses	0	0	0
Deductions	(15)	(10)	(9)
Balance at End of Year	8	23	33
Deferred tax asset valuation allowance
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	34,889	21,861	18,248
Additions Charged to Cost and Expenses	3,950	13,028	3,613
Deductions	0	0	0
Balance at End of Year	$ 38,839	$ 34,889	$ 21,861