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
	Mar. 31, 2018	Mar. 31, 2017
Pension Benefits
Plan expense for noncontributory benefit pension plan and post-retirement health and life plans
Service cost	$ 0	$ 0
Interest cost	266	293
Expected return on plan assets	(518)	(484)
Recognized actuarial loss	87	100
Settlement/curtailment/termination gains	0	0
Net periodic benefit (return) cost	(165)	(91)
Other Benefits
Plan expense for noncontributory benefit pension plan and post-retirement health and life plans
Service cost	2	3
Interest cost	9	13
Expected return on plan assets	0	0
Recognized actuarial loss	0	13
Settlement/curtailment/termination gains	0	0
Net periodic benefit (return) cost	$ 11	$ 29