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 and qualifying accounts
Bad debt expense, net of recoveries	$ 3,700	$ 2,800	$ 3,300
Allowance for doubtful service accounts receivable (deducted from accounts receivable)
Movement in valuation and qualifying accounts
Balance at Beginning of Period	1,734	1,757	1,707
Charged to Costs and Expenses	4,835	3,840	4,746
Deductions	(5,070)	(3,863)	(4,696)
Balance at End of Period	1,499	1,734	1,757
Deferred tax valuation allowance
Movement in valuation and qualifying accounts
Balance at Beginning of Period	110,194	113,392	245,104
Charged to Costs and Expenses	20,102	3,102	1,058
Deductions	(623)	(6,300)	(132,770)
Balance at End of Period	$ 129,673	$ 110,194	$ 113,392