Employee Benefit Plans - Accumulated and projected benefit obligations in excess of plan assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Pensions
Defined Benefit Plan Disclosure [Line Items]
Aggregate accumulated benefit obligation	$ 4,108	$ 3,722
Aggregate fair value of plan assets	3,449	3,672
Aggregate projected benefit obligation	13,834	12,334
Aggregate fair value of plan assets	11,954	11,370
Other post-retirement benefits
Defined Benefit Plan Disclosure [Line Items]
Aggregate accumulated benefit obligation	13,211	13,061
Aggregate fair value of plan assets	0	0
Aggregate projected benefit obligation	13,211	13,061
Aggregate fair value of plan assets	$ 0	$ 0