Schedule II Valuation of Qualifying Accounts and Reserves (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Valuation Allowances and Reserves [Roll Forward]
Valuation Allowances and Reserves, Balance Beginning of Period	$ 1,475	$ 1,475	$ 1,262
Valuation Allowances and Reserves, Charged to Cost and Expense	0	0	318
Valuation Allowances and Reserves, Charged to Other Accounts	0	0	0
Valuation Allowances and Reserves, Deductions	0	0	105
Valuation Allowances and Reserves, Balance End of Period	$ 1,475	$ 1,475	$ 1,475