Schedule II - Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Valuation Allowances and Reserves [Roll Forward]
Additions Charged to Cost and Expenses	$ (13,028)	$ (3,613)	$ (4,604)
Income tax provision	21,681	5,064	(859)
Allowance for doubtful accounts
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	33	42	42
Additions Charged to Cost and Expenses	0	0	0
Deductions	(10)	(9)	0
Balance at End of Year	23	33	42
Deferred tax asset valuation allowance
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	21,861	18,248	13,644
Additions Charged to Cost and Expenses	13,028	3,613	4,604
Deductions	0	0	0
Balance at End of Year	$ 34,889	$ 21,861	$ 18,248