Employee Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Pension Plan
Net periodic benefit cost
Service cost	$ 1,714	$ 1,623	$ 3,427	$ 3,246
Interest cost	1,765	1,663	3,530	3,326
Expected return on plan assets	(2,412)	(1,988)	(4,824)	(3,976)
Amortization of prior service credit	0	0	0	0
Amortization of net loss	891	992	1,782	1,984
Net periodic benefit cost	1,958	2,290	3,915	4,580
Estimated 2017 pension plan contribution, disclosed in prior year 10K					$ 6,400
Contribution by employer			3,196
Postretirement Benefit Plan
Net periodic benefit cost
Service cost	505	932	1,010	1,864
Interest cost	482	754	964	1,508
Expected return on plan assets	0	0	0	0
Amortization of prior service credit	(1,352)	0	(2,704)	0
Amortization of net loss	461	379	922	758
Net periodic benefit cost	$ 96	$ 2,065	$ 192	$ 4,130