Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Components of net periodic benefit cost:
Service cost	$ 3,930	$ 3,668	$ 4,579
Interest cost	9,729	12,261	11,480
Expected return on plan assets	(16,357)	(15,699)	(16,389)
Amortization of prior service cost (credit)	190	169	(42)
Settlement loss (gain)	3,153	(7)	1,342
Net loss (gain) recognition	2,930	1,432	2,775
Net periodic benefit cost	3,575	1,824	3,745
Other Benefits
Components of net periodic benefit cost:
Service cost	33	35	47
Interest cost	809	960	945
Expected return on plan assets	0	0	0
Amortization of prior service cost (credit)	0	0	0
Settlement loss (gain)	0	0	0
Net loss (gain) recognition	(59)	(133)	(12)
Net periodic benefit cost	$ 783	$ 862	$ 980