Employee Benefits (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Pension Plan
Net periodic benefit cost
Service cost		$ 1,714	$ 1,623	$ 5,141	$ 4,869
Interest cost		1,765	1,663	5,295	4,989
Expected return on plan assets		(2,413)	(1,988)	(7,237)	(5,964)
Amortization of prior service credit		0	0	0	0
Amortization of net loss		891	992	2,673	2,976
Net periodic benefit cost		1,957	2,290	5,872	6,870
Estimated 2017 pension plan contribution, disclosed in prior year 10K						$ 6,400
Contribution by employer	$ 5,000			11,396
Postretirement Benefit Plan
Net periodic benefit cost
Service cost		505	932	1,515	2,796
Interest cost		482	754	1,446	2,262
Expected return on plan assets		0	0	0	0
Amortization of prior service credit		(1,352)	0	(4,056)	0
Amortization of net loss		462	379	1,384	1,137
Net periodic benefit cost		$ 97	$ 2,065	$ 289	$ 6,195