Employee Benefit Plans - Components of Net Periodic Pension/Postretirement Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the year	$ 2	$ 3	$ 4
Interest cost on projected benefit obligations	1,228	1,711	1,769
Expected return on plan assets	1,596	2,199	2,151
Amortization of net loss	525	575	656
Settlement expense	641	1,510	762
Net periodic pension/postretirement benefit cost	800	1,600	1,040
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the year	121	137	147
Interest cost on projected benefit obligations	195	257	251
Net gain	(49)	0	0
Amortization of prior service (credit) cost	(25)	(25)	53
Net periodic pension/postretirement benefit cost	$ 242	$ 369	$ 451