Pension and Other Postretirement Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Pension Benefits
Components of net periodic benefit cost (income)
Service cost	$ 2,462	$ 1,880	$ 4,778	$ 4,122
Interest cost	2,538	2,808	5,050	5,788
Expected return on plan assets	(2,902)	(3,257)	(5,805)	(6,556)
Settlement expense	3,850	1,775	3,850	1,775
Amortization of net loss (gain)	1,343	129	2,307	401
Amortization of prior service cost (credit)	69	48	139	92
Net periodic benefit cost (income)	7,360	3,383	10,319	5,622
Postretirement Benefits
Components of net periodic benefit cost (income)
Service cost	129	126	300	287
Interest cost	104	140	216	278
Expected return on plan assets	0	0	0	0
Settlement expense	0	0	0	0
Amortization of net loss (gain)	(270)	(355)	(484)	(718)
Amortization of prior service cost (credit)	(34)	(34)	(68)	(68)
Net periodic benefit cost (income)	$ (71)	$ (123)	$ (36)	$ (221)