Employee Benefit and Deferred Compensation Plans - Plan Expense for Non-Contributory Benefit Pension Plan and Post-Retirement Health and Life Plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Benefits
Plan expense for noncontributory benefit pension plan and post-retirement health and life plans
Service cost	$ 0	$ 0	$ 0	$ 0
Interest cost	256	291	522	584
Expected return on plan assets	(520)	(487)	(1,038)	(971)
Recognized actuarial loss	77	100	164	200
Net periodic benefit (return) cost	(187)	(96)	(352)	(187)
Other Benefits
Plan expense for noncontributory benefit pension plan and post-retirement health and life plans
Service cost	2	1	4	4
Interest cost	7	8	16	21
Expected return on plan assets	0	0	0	0
Recognized actuarial loss	0	(10)	0	3
Net periodic benefit (return) cost	$ 9	$ (1)	$ 20	$ 28