Pension and Employee Benefit Plans - Components of net periodic benefit cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Modified Retirement Plan | Unfunded Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 363	$ 551	$ 382
Interest cost	801	778	740
Expected return on plan assets	0	0	0
Amortization of prior service cost	666	560	501
Amortization of net losses	241	238	167
Net periodic benefit cost	2,071	2,127	1,790
Funded Plan | Funded Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0
Interest cost	647	738	842
Expected return on plan assets	(551)	(630)	(696)
Amortization of prior service cost	0	0	0
Amortization of net losses	0	0	0
Net periodic benefit cost	$ 96	$ 108	$ 146