Employee Benefit Plans - Components of Net Periodic Pension/Postretirement Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost — benefits earned during the year	$ 3	$ 4	$ 4
Interest cost on projected benefit obligations	1,711	1,769	1,888
Expected return on plan assets	(2,199)	(2,151)	(2,806)
Amortization of net loss	575	656	606
Settlement expense	1,510	762	858
Net periodic pension/postretirement benefit cost	1,600	1,040	550
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost — benefits earned during the year	137	147	130
Interest cost on projected benefit obligations	257	251	269
Amortization of prior service (credit) cost	(25)	53	161
Net periodic pension/postretirement benefit cost	$ 369	$ 451	$ 560