Employee Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Pension Plan
Net periodic benefit cost
Service cost	$ 2,707	$ 1,997	$ 8,122	$ 5,991
Interest cost	2,066	2,080	6,199	6,240
Expected return on plan assets	(3,385)	(2,696)	(10,154)	(8,088)
Amortization of prior service credit	0	0	0	0
Amortization of net loss	979	901	2,936	2,703
Net periodic benefit cost	2,367	2,282	7,103	6,846
Postretirement Benefit Plan
Net periodic benefit cost
Service cost	432	456	1,296	1,368
Interest cost	253	319	760	956
Expected return on plan assets	0	0	0	0
Amortization of prior service credit	(2,021)	(2,221)	(6,063)	(6,463)
Amortization of net loss	94	224	282	671
Net periodic benefit cost	$ (1,242)	$ (1,222)	(3,725)	$ (3,468)
Estimated 2020 pension plan contribution, disclosed in prior year 10K					$ 10,000
Contribution by employer			$ 10,000