Employee Benefits (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Pension Plan
Net periodic benefit cost
Service cost	$ 2,175	$ 1,714
Interest cost	1,875	1,765
Expected return on plan assets	(2,625)	(2,412)
Amortization of prior service credit	0	0
Amortization of net loss	1,071	890
Net periodic benefit cost	2,496	1,957
Estimated 2018 pension plan contribution, disclosed in prior year 10K			$ 6,400
Contribution by employer	1,600
Postretirement Benefit Plan
Net periodic benefit cost
Service cost	749	505
Interest cost	502	482
Expected return on plan assets	0	0
Amortization of prior service credit	(1,352)	(1,352)
Amortization of net loss	589	461
Net periodic benefit cost	$ 488	$ 96