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
Movement in Valuation Allowances and Reserves
Balance at Beginning of Year	$ 1,417	$ 1,485	$ 1,036
Additions Charged to Costs and Expenses	1,517	982	1,113
Deductions	783	1,050	664
Balance at End of Year	2,151	1,417	1,485
Valuation allowance for deferred tax assets
Movement in Valuation Allowances and Reserves
Balance at Beginning of Year	29,719	0
Additions Charged to Costs and Expenses	0	29,719
Deductions	6,023	0
Balance at End of Year	$ 23,696	$ 29,719	$ 0