Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits
Components of net periodic benefit cost:
Service cost	$ 3,668	$ 4,579	$ 4,767
Interest cost	12,261	11,480	7,551
Expected return on plan assets	(15,699)	(16,389)	(10,642)
Amortization of prior service cost (credit)	169	(42)	(41)
Settlement loss (gain)	(7)	1,342	(8)
Net loss (gain) recognition	1,432	2,775	2,562
Net periodic benefit cost	1,824	3,745	4,189
Other Benefits
Components of net periodic benefit cost:
Service cost	35	47	49
Interest cost	960	945	1,139
Expected return on plan assets	0		0
Amortization of prior service cost (credit)	0		0
Settlement loss (gain)	0		0
Net loss (gain) recognition	(133)	(12)	0
Net periodic benefit cost	$ 862	$ 980	$ 1,188