Employee Benefit Plans - Narrative (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Jun. 30, 2015 USD ($)	Dec. 31, 2017 USD ($) participant	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cost recognized		$ 1,299	$ 1,097	$ 1,032
Effect of one percentage point increase in assumed health care costs, affect on accumulated benefit obligation		98
Effect of one percentage point decrease in assumed health care costs, effect on accumulated benefit obligation		(93)
Effect of one percentage point increase in assumed health care costs, affect on service and interest cost		5
Effect of one percentage point decrease in assumed health care costs, affect on service and interest cost		$ (4)
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contributions	$ 741
Post-employment benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Number of participants | participant		177
Number of active participants eligible for benefits | participant		25
Minimum age of participant to become eligible for benefits		62 years
Requisite service period		5 years
Age up to which health benefits continuation paid		65 years
Expected contributions in next fiscal year, net of Medicare Part D subsidy receipts		$ 486
Medicare Part D subsidy receipts to the plan in next fiscal year		486
Post-employment benefits | Medicare Part D Subsidy Receipts
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Medicare Part D subsidy receipts to the plan in next fiscal year		$ 15