Employee Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost — benefits earned during the period	$ 141	$ 137	$ 423	$ 411
Interest cost on projected benefit obligation	1,262	1,355	3,794	4,065
Expected return on assets	(1,356)	(1,519)	(4,068)	(4,557)
Amortization of net loss (gain)	503	464	1,506	1,392
Net expense	550	437	1,655	1,311
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost — benefits earned during the period	2	1	4	3
Interest cost on projected benefit obligation	92	97	276	291
Expected return on assets	0	0	0	0
Amortization of net loss (gain)	(14)	(19)	(40)	(57)
Net expense	$ 80	$ 79	$ 240	$ 237