Employee Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Pension Plan
Net periodic benefit cost
Service cost	$ 2,707	$ 1,997	$ 5,414	$ 3,994
Interest cost	2,066	2,080	4,132	4,160
Expected return on plan assets	(3,385)	(2,696)	(6,770)	(5,392)
Amortization of prior service credit	0	0	0	0
Amortization of net loss	979	901	1,958	1,802
Net periodic benefit cost	2,367	2,282	4,734	4,564
Postretirement Benefit Plan
Net periodic benefit cost
Service cost	432	456	864	912
Interest cost	253	318	506	637
Expected return on plan assets	0	0	0	0
Amortization of prior service credit	(2,021)	(2,221)	(4,042)	(4,242)
Amortization of net loss	94	224	188	447
Net periodic benefit cost	$ (1,242)	$ (1,223)	(2,484)	$ (2,246)
Estimated 2020 pension plan contribution, disclosed in prior year 10K					$ 10,000
Contribution by employer			$ 5,000