Pension and Employee Benefit Plans - Components of net periodic benefit cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Modified Retirement Plan | Unfunded Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 392	$ 363	$ 551
Interest cost	931	801	778
Expected return on plan assets	0	0	0
Amortization of prior service cost	635	666	560
Amortization of net losses	59	241	238
Net periodic benefit cost	2,017	2,071	2,127
Funded Plan | Funded Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0
Interest cost	166	647	738
Expected return on plan assets	(106)	(551)	(630)
Amortization of prior service cost	0	0	0
Amortization of net losses	0	0	0
Net periodic benefit cost	$ 60	$ 96	$ 108