Employee Benefit Plans - Fair Value Measurements (Details) - Pension - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Investments at Fair Value:	$ 57,377	$ 51,114	$ 50,970
Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Investments at Fair Value:	8,986	13,962
Mutual Funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Investments at Fair Value:	8,986	13,962
Mutual Funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Investments at Fair Value:	0	0
Mutual Funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Investments at Fair Value:	0	0
Common Collective Trusts
Defined Benefit Plan Disclosure [Line Items]
Investments at Net Asset Value:	$ 48,391	$ 37,152