Employee Benefit Plans - Fair Value of Pension Plan Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 7,556	$ 7,647	$ 8,428	$ 10,949
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,853	3,725
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,703	3,922
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	617	780
Cash | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	617	780
Cash | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Cash | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Government sponsored entities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,892	1,496
Government sponsored entities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Government sponsored entities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,892	1,496
Government sponsored entities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Municipal bonds and notes
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,931	1,453
Municipal bonds and notes | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Municipal bonds and notes | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,931	1,453
Municipal bonds and notes | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Corporate bonds and notes
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	880	973
Corporate bonds and notes | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Corporate bonds and notes | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	880	973
Corporate bonds and notes | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,768	2,505
U.S. companies | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,768	2,505
U.S. companies | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. companies | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Foreign companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	468	440
Foreign companies | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	468	440
Foreign companies | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Foreign companies | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0	$ 0