Pension and Other Postretirement Benefits - Amounts Recognized in Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Amounts recognized in the balance sheets:
Accrued benefit liability (noncurrent)	$ (132,791)	$ (102,085)
Pension Benefits
Amounts recognized in the balance sheets:
Prepaid benefit cost	4,801	3,174
Accrued benefit liability (current)	(3,320)	(3,736)
Accrued benefit liability (noncurrent)	(108,054)	(73,307)
Net funded status	(106,573)	(73,869)
Other Benefits
Amounts recognized in the balance sheets:
Prepaid benefit cost	0	0
Accrued benefit liability (current)	(1,405)	(1,555)
Accrued benefit liability (noncurrent)	(24,738)	(28,778)
Net funded status	$ (26,143)	$ (30,333)