Employee Benefit Plans - Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension
Defined Benefit Plan Disclosure [Line Items]
Net (losses) gains	$ (583)	$ 3,119	$ (477)
Amortization of losses (gains)	466	2,526	3,733
Amortization of prior service cost	0	0	13
Postretirement
Defined Benefit Plan Disclosure [Line Items]
Net (losses) gains	(89)	(99)	123
Amortization of losses (gains)	(1)	(13)	12
Amortization of prior service cost	$ 0	$ 0	$ 0