Employee Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Pension Plan
Net periodic benefit cost
Service cost	$ 2,175	$ 1,714	$ 4,350	$ 3,427
Interest cost	1,875	1,765	3,750	3,530
Expected return on plan assets	(2,626)	(2,412)	(5,251)	(4,824)
Amortization of prior service credit	0	0	0	0
Amortization of net loss	1,072	891	2,143	1,782
Net periodic benefit cost	2,496	1,958	4,992	3,915
Estimated 2018 pension plan contribution, disclosed in prior year 10K					$ 6,400
Contribution by employer			3,200
Postretirement Benefit Plan
Net periodic benefit cost
Service cost	750	505	1,499	1,010
Interest cost	502	482	1,004	964
Expected return on plan assets	0	0	0	0
Amortization of prior service credit	(1,352)	(1,352)	(2,704)	(2,704)
Amortization of net loss	589	461	1,178	922
Net periodic benefit cost	$ 489	$ 96	$ 977	$ 192