Employee Benefit Plans - Components of Net Periodic Benefit Cost (Details) - Postretirement Benefit Plans - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Components of net periodic benefit cost:
Net periodic (income) expense	$ (64)	$ (99)
General and administrative
Components of net periodic benefit cost:
Service cost	16	18
Interest expense
Components of net periodic benefit cost:
Interest cost	53	57
Other income
Components of net periodic benefit cost:
Expected long-term return on assets	(51)	(62)
Prior service cost amortization	(113)	(114)
Actuarial loss amortization	$ 31	$ 2