Employee Benefit Plans - Reconciliation of Benefit Obligations, Plan Assets and Funded Status (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other postretirement benefits
Change in benefit obligation:
Benefit obligation at beginning of plan year	$ 6,131	$ 6,100
Service cost	121	137	$ 147
Interest cost	195	257	251
Actuarial loss	(180)	(249)
Plan participants' contributions	564	604
Benefits paid	(758)	(718)
Projected benefit obligation at measurement date	6,073	6,131	6,100
Pension plans
Change in benefit obligation:
Benefit obligation at beginning of plan year	42,030	45,471
Service cost	2	3	4
Interest cost	1,228	1,711	1,769
Actuarial loss	2,940	537
Benefits paid	(3,302)	(5,692)
Projected benefit obligation at measurement date	42,898	42,030	45,471
Change in plan assets:
Fair value of plan assets at beginning of plan year	42,061	43,603
Actual return on plan assets	1,933	3,150
Company contribution	825	1,000
Benefits paid	(3,302)	(5,692)
Fair value of plan assets at measurement date	41,517	42,061	$ 43,603
(Pension liability) prepaid pension asset	$ (1,381)	$ 31