Employee Benefit Plans - Components of Net Periodic Benefit Cost and Other Amounts Recognized in Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Components of Net Periodic Postretirement Benefit Cost
Service cost	$ 1,533	$ 1,776	$ 1,508
Interest cost	1,283	1,172	1,097
Amortization of prior service cost	(709)	(709)	(1,183)
Net periodic postretirement cost	2,107	2,239	1,422
Recognized curtailment gain	0	0	(4,917)
Total post retirement cost (income)	2,107	2,239	(3,495)
Other Changes in Benefit Obligations Recognized in Other Comprehensive Income (Loss)
Net (gain) loss	3,279	(3,165)	4,178
Prior service cost	0	0	(8,643)
Amortization of prior service cost	709	709	2,733
Total recognized in other comprehensive income	3,988	(2,456)	(1,732)
Total recognized in net periodic benefit cost (income) and other comprehensive income	$ 6,095	$ (217)	$ (5,227)