Employee Benefit Plans Summary of Pension Plan and Postretirement Plan Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 696	$ 830	$ 1,527	$ 1,660
Expected return on plan assets	(573)	(451)	(1,024)	(902)
Net amortization and deferral	551	663	1,215	1,326
Net periodic benefit	674	1,042	1,718	2,084
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	12	17	29	34
Net accretion and deferral	(139)	(141)	(280)	(282)
Net periodic benefit	$ (127)	$ (124)	$ (251)	$ (248)