Employee Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Defined pension plan benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost – benefits earned during the year	$ 29	$ 24
Interest cost on projected benefit obligations	95	318
Expected return on plan assets	(244)	(402)
Amortization of prior service cost	0	0
Amortization of net loss	88	135
Net periodic benefit cost	(32)	75
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost – benefits earned during the year	34	35
Interest cost on projected benefit obligations	48	51
Expected return on plan assets	0	0
Amortization of prior service cost	(6)	(6)
Amortization of net loss	0	0
Net periodic benefit cost	$ 76	$ 80