Pension and Other Postretirement Benefits (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension Benefits
Components of net periodic benefit cost (income)
Service cost	$ 2,316	$ 2,242
Interest cost	2,512	2,980
Expected return on plan assets	(2,903)	(3,299)
Amortization of net loss (gain)	964	272
Amortization of prior service cost (credit)	70	44
Net periodic benefit cost (income)	2,959	2,239
Postretirement Benefits
Components of net periodic benefit cost (income)
Service cost	171	161
Interest cost	112	138
Expected return on plan assets	0	0
Amortization of net loss (gain)	(214)	(363)
Amortization of prior service cost (credit)	(34)	(34)
Net periodic benefit cost (income)	$ 35	$ (98)