Accounts Receivable - Activity in Accounts Receivable Reserves (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning balance	$ 6,145	$ 7,364	$ 9,023
Charges to income from operations	5,809	49,677	37,870
Collections from cash basis customers	(10,675)	(50,896)	(39,529)
Ending balance	$ 1,279	$ 6,145	$ 7,364