Schedule II - Valuation and Qualifying Accounts and Reserves (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	$ 14,372	$ 9,997	$ 12,455
Additions charged to expense	9,770	6,797	430
Deductions	(8,157)	(2,422)	(2,888)
Balance at end of year	15,985	14,372	9,997
Valuation allowance for deferred tax asset
Movement in Valuation Allowances and Reserves
Balance at beginning of year	127,095	132,624	116,935
Additions charged to expense	53,782	6,523	21,232
Deductions	(2,721)	(12,052)	(5,543)
Balance at end of year	$ 178,156	$ 127,095	$ 132,624