Organization and Significant Accounting Policies - Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Doubtful Accounts Receivable [Roll Forward]
Balance at January 1	$ 1,569	$ 1,772	$ 1,922
Provision charged to expense	0	172	0
Write-offs / Recoveries	(24)	(375)	(150)
Balance at December 31	$ 1,545	$ 1,569	$ 1,772