Employee Benefit Plans - Changes in Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of Net Periodic Postretirement Benefit Cost
Service cost	$ 1,508	$ 2,323	$ 1,808
Interest cost	1,097	1,498	1,448
Amortization of prior service cost	(1,183)	111	0
Net periodic postretirement cost	1,422	3,932	3,256
Recognized curtailment gain	(4,917)	0	0
Total post retirement cost (income)	(3,495)	3,932	3,256
Other Changes in Benefit Obligations Recognized in Other Comprehensive Income (Loss)
Net (gain) loss	4,178	(2,846)	(2,829)
Prior service cost	(8,643)	815	0
Amortization of prior service cost	2,733	(111)	0
Total recognized in other comprehensive income	(1,732)	(2,142)	(2,829)
Total recognized in net periodic benefit cost (income) and other comprehensive income	$ (5,227)	$ 1,790	$ 427