Employee Benefit Plans - Changes in Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Components of Net Periodic Postretirement Benefit Cost
Service cost	$ 1,776	$ 1,508	$ 2,323
Interest cost	1,172	1,097	1,498
Amortization of prior service cost	(709)	(1,183)	111
Net periodic postretirement cost	2,239	1,422	3,932
Recognized curtailment gain	0	(4,917)	0
Total post retirement cost (income)	2,239	(3,495)	3,932
Other Changes in Benefit Obligations Recognized in Other Comprehensive Income (Loss)
Net (gain) loss	(3,165)	4,178	(2,846)
Prior service cost	0	(8,643)	815
Amortization of prior service cost	709	2,733	(111)
Total recognized in other comprehensive income	(2,456)	(1,732)	(2,142)
Total recognized in net periodic benefit cost (income) and other comprehensive income	$ (217)	$ (5,227)	$ 1,790