Employee Benefit Plans - Other changes in the plan assets and benefit obligation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Other changes recognized in accumulated other comprehensive income (loss)
Pension liability in other comprehensive income (loss), beginning of year	$ 2,750
Plan change	1,276	$ 506
Net prior service cost	(140)
Net loss	4,564	2,244
Pension liability in other comprehensive income (loss), end of year	8,450	$ 2,750
Amortization into next prior service cost over the next fiscal year	200
Amortization into next actuarial loss over the next fiscal year	$ 300