Retirement Plans - Fair Value of Plan Assets by Investment Type (Details) - Pension plan - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 7,819	$ 6,974	$ 6,707
Cash and Cash Equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	455	776
Collective Funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,942	3,423
Treasury/Corporate Notes | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,583	1,181
Corporate Equities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 1,839	$ 1,594