Changes in Operations and Non-operating Items Changes in Accrued Liability for Severance and Related Expenses (Details) - Employee Severance - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Severance and Related Activity Liability [Roll Forward]
Beginning balance	$ 8,981	$ 3,595
Additions	4,739	10,406
Payments	(9,144)	(5,020)
Ending balance	$ 4,576	$ 8,981