Employee Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined pension plan benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost (income) – benefits earned during the year	$ 41	$ (11)	$ 70	$ 13
Interest cost on projected benefit obligations	222	469	317	787
Expected return on plan assets	(591)	(396)	(835)	(798)
Amortization of prior service cost	0	0	0	0
Amortization of net loss (benefit)	201	141	289	276
Settlement expense	8,168	0	8,168	0
Net periodic benefit cost	8,041	203	8,009	278
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost (income) – benefits earned during the year	28	28	62	63
Interest cost on projected benefit obligations	47	66	95	117
Expected return on plan assets	0	0	0	0
Amortization of prior service cost	0	(7)	(7)	(13)
Amortization of net loss (benefit)	(15)	(24)	(15)	(24)
Settlement expense	0	0	0	0
Net periodic benefit cost	$ 60	$ 63	$ 135	$ 143