Pension and Other Postretirement Benefits - Amounts Recognized in Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Amounts recognized in the balance sheets:
Accrued benefit liability (noncurrent)	$ (102,085)	$ (104,050)
Pension Benefits [Member]
Amounts recognized in the balance sheets:
Prepaid benefit cost	3,174	3,148
Accrued benefit liability (current)	(3,736)	(3,022)
Liabilities held for sale	0	(447)
Accrued benefit liability (noncurrent)	(73,307)	(73,790)
Net funded status	(73,869)	(74,111)
Other Benefits [Member]
Amounts recognized in the balance sheets:
Prepaid benefit cost	0	0
Accrued benefit liability (current)	(1,555)	(1,778)
Liabilities held for sale	0	0
Accrued benefit liability (noncurrent)	(28,778)	(30,260)
Net funded status	$ (30,333)	$ (32,038)