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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 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	261	292	783	876
Expected return on plan assets	(520)	(485)	(1,558)	(1,456)
Recognized actuarial loss	82	101	246	301
Net periodic benefit (return) cost	(177)	(92)	(529)	(279)
Other Benefits
Plan expense for noncontributory benefit pension plan and post-retirement health and life plans
Service cost	2	2	6	6
Interest cost	7	11	23	32
Expected return on plan assets	0	0	0	0
Recognized actuarial loss	0	2	0	5
Net periodic benefit (return) cost	$ 9	$ 15	$ 29	$ 43