Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for doubtful receivables
Changes in valuation and qualifying Accounts
Balance at the beginning of the period	$ 574	$ 0	$ 0
Charged to Costs and Expenses	77	574	0
Charged To Other Accounts	0	0	0
Deductions From Reserves	(651)	0	0
Balance at the end of the period	0	574	0
Allowance for deferred tax assets
Changes in valuation and qualifying Accounts
Balance at the beginning of the period	87,517	116,541	75,941
Charged to Costs and Expenses	6,008	(29,024)	40,600
Charged To Other Accounts	0	0	0
Deductions From Reserves	0	0	0
Balance at the end of the period	$ 93,525	$ 87,517	$ 116,541