Employee Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Pension Plan
Net periodic benefit cost
Service cost	$ 1,997	$ 2,175	$ 3,994	$ 4,350
Interest cost	2,080	1,875	4,160	3,750
Expected return on plan assets	(2,696)	(2,626)	(5,392)	(5,251)
Amortization of prior service credit	0	0	0	0
Amortization of net loss	901	1,072	1,802	2,143
Net periodic benefit cost	2,282	2,496	4,564	4,992
Estimated 2019 pension plan contribution, disclosed in prior year 10K					$ 4,000
Contribution by employer			2,000
Postretirement Benefit Plan
Net periodic benefit cost
Service cost	456	750	912	1,499
Interest cost	318	502	637	1,004
Expected return on plan assets	0	0	0	0
Amortization of prior service credit	(2,221)	(1,352)	(4,242)	(2,704)
Amortization of net loss	224	589	447	1,178
Net periodic benefit cost	$ (1,223)	$ 489	$ (2,246)	$ 977