Employee Benefit Plans - Amounts Recognized in the Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Amounts Recognized in the Balance Sheet
Long-term liabilities	$ 36,259	$ 35,293
Postretirement Benefits Other than Pensions
Amounts Recognized in the Balance Sheet
Current liabilities	1,223	1,333	$ 1,249
Long-term liabilities	36,259	35,293	35,827
Amounts recognized in the balance sheet	$ 37,482	$ 36,626	$ 37,076