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, 2020	Dec. 31, 2019	Dec. 31, 2018
Components of Net Periodic Postretirement Benefit Cost
Service cost	$ 1,493	$ 1,533	$ 1,776
Interest cost	974	1,283	1,172
Amortization of prior service cost	(709)	(709)	(709)
Amortization of net loss	(36)	0	0
Net periodic postretirement cost	1,722	2,107	2,239
Other Changes in Benefit Obligations Recognized in Other Comprehensive Income (Loss)
Net (gain) loss	(2,118)	3,279	(3,165)
Amortization of prior service cost	709	709	709
Amortization of net loss	36	0	0
Total recognized in other comprehensive income	(1,373)	3,988	(2,456)
Total recognized in net periodic benefit cost (income) and other comprehensive income	$ 349	$ 6,095	$ (217)