Employee Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Defined pension plan benefits
Defined Benefit Plan Disclosure [Line Items]
Service (income) cost – benefits earned during the year	$ (11)	$ 48	$ 13	$ 55
Interest cost on projected benefit obligations	469	450	787	905
Expected return on plan assets	(396)	(538)	(798)	(1,087)
Amortization of prior service cost	0	0	0	0
Amortization of net loss (benefit)	141	154	276	311
Net periodic benefit cost	203	114	278	184
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Service (income) cost – benefits earned during the year	28	33	63	69
Interest cost on projected benefit obligations	66	65	117	129
Expected return on plan assets	0	0	0	0
Amortization of prior service cost	(7)	(7)	(13)	(13)
Amortization of net loss (benefit)	(24)	0	(24)	0
Net periodic benefit cost	$ 63	$ 91	$ 143	$ 185