Employee Benefit Plans - Components of Net Period Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 18	$ 17	$ 46	$ 52
Interest cost	606	726	1,815	2,180
Expected return on plan assets	(886)	(751)	(2,651)	(2,257)
Amortization of net loss	407	517	1,218	1,551
Amortization of prior service cost	7	8	22	25
Net pension benefit cost	152	517	450	1,551
Other Post-Retirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	0
Interest cost	4	5	11	17
Expected return on plan assets	0	0	0	0
Amortization of net loss	0	(1)	0	(3)
Amortization of prior service cost	0	0	0	0
Net pension benefit cost	4	4	11	14
Retirement Income Plan | Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	0
Interest cost	315	397	946	1,192
Expected return on plan assets	(500)	(440)	(1,501)	(1,320)
Amortization of net loss	255	377	763	1,132
Amortization of prior service cost	0	0	0	0
Net pension benefit cost	70	334	208	1,004
National Service-Related Pension Plan | Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	0
Interest cost	279	312	838	934
Expected return on plan assets	(366)	(294)	(1,099)	(882)
Amortization of net loss	152	141	455	422
Amortization of prior service cost	0	0	0	0
Net pension benefit cost	65	159	194	474
Superannuation | Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	18	17	46	52
Interest cost	8	12	20	37
Expected return on plan assets	(20)	(17)	(51)	(55)
Amortization of net loss	0	0	0	0
Amortization of prior service cost	7	8	22	25
Net pension benefit cost	$ 13	$ 20	$ 37	$ 59