Employee Benefit Plans - Reconciliation of Benefit Obligations, Plan Assets and Funded Status (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension plans
Change in benefit obligation:
Benefit obligation at beginning of year	$ 45,471	$ 48,200
Service cost	3	4	$ 4
Interest cost	1,711	1,769	1,888
Actuarial loss/(gain)	537	(906)
Benefits paid	(5,692)	(3,596)
Benefit obligation and funded status at end of year	42,030	45,471	48,200
Change in plan assets:
Fair value of plan assets at beginning of plan year	43,603	48,656
Actual return on plan assets	3,150	(1,457)
Company contribution	1,000	0
Benefits paid	(5,692)	(3,596)
Fair value of plan assets at measurement date	42,061	43,603	48,656
Funded status of the plan	31	(1,868)
Prepaid pension asset	31
Accrued pension liability		(1,868)
Other postretirement benefits
Change in benefit obligation:
Benefit obligation at beginning of year	6,100	6,767
Service cost	137	147	130
Interest cost	257	251	269
Actuarial loss/(gain)	(249)	(812)
Plan participants' contributions	604	628
Benefits paid	(718)	(881)
Benefit obligation and funded status at end of year	6,131	6,100	$ 6,767
Change in plan assets:
Benefits paid	(718)	(881)
Accrued pension liability	$ (6,131)	$ (6,100)