Employee Benefit Plans - Change in PBO and Assets and Reconciliations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent liabilities	$ (31,905)	$ (31,856)
Pension
Change in Projected Benefit Obligation
Projected benefit obligation at beginning of year	81,752	84,005
Service cost	0	1,307	$ 1,484
Interest cost	3,259	3,474	3,319
Curtailment gain	0	(5,447)
Actuarial loss	6,123	1,296
Benefits paid	(3,148)	(2,883)
Projected benefit obligation at end of year	87,986	81,752	84,005
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	51,114	50,970
Actual return on plan assets	9,909	3,557
Employer contributions	90	29
Benefits paid	(3,148)	(2,883)
Other expense	(588)	(559)
Fair value of plan assets at end of year	57,377	51,114	50,970
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Net accrued benefit cost	(30,609)	(30,638)
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Current liabilities	(92)	(36)
Noncurrent liabilities	(30,517)	(30,602)
Net amount recognized	(30,609)	(30,638)
Postretirement
Change in Projected Benefit Obligation
Projected benefit obligation at beginning of year	1,285	1,159
Service cost	6	4	11
Interest cost	53	42	52
Curtailment gain	0	0
Actuarial loss	89	99
Benefits paid	(13)	(19)
Projected benefit obligation at end of year	1,420	1,285	1,159
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Employer contributions	13	19
Benefits paid	(13)	(19)
Other expense	0	0
Fair value of plan assets at end of year	0	0	$ 0
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Net accrued benefit cost	(1,420)	(1,285)
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Current liabilities	(32)	(30)
Noncurrent liabilities	(1,388)	(1,255)
Net amount recognized	$ (1,420)	$ (1,285)