Selected Balance Sheet Data, Accounts Receivable (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Accounts Receivable, Net
Accounts receivable		$ 68,073	$ 58,896
Less allowance for bad debts		(4,873)	(4,865)
Accounts receivable, net	[1]	$ 63,200	$ 54,031

[1] Accounts receivable, net includes $3.4 million and an immaterial amount due from affiliates for 2017 and 2016, respectively.