Employee Benefit Plans - Components of Net Period Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 59	$ 78	$ 246
Interest cost	2,417	2,907	2,741
Expected return on plan assets	(3,533)	(3,010)	(3,588)
Amortization of net loss	1,624	2,069	1,959
Amortization of prior service cost	27	28	30
Effect of settlement	134	(10)	68
Net pension benefit cost	728	2,062	1,456
Pension | Retirement Income Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	31
Interest cost	1,261	1,590	1,418
Expected return on plan assets	(2,002)	(1,760)	(2,047)
Amortization of net loss	1,017	1,509	1,244
Amortization of prior service cost	0	0	0
Effect of settlement	134	0	0
Net pension benefit cost	410	1,339	646
Pension | National Service-Related Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	78
Interest cost	1,117	1,245	1,199
Expected return on plan assets	(1,465)	(1,176)	(1,369)
Amortization of net loss	607	564	715
Amortization of prior service cost	0	0	0
Effect of settlement	0	0	0
Net pension benefit cost	259	633	623
Pension | Superannuation
Defined Benefit Plan Disclosure [Line Items]
Service cost	59	78	137
Interest cost	25	49	104
Expected return on plan assets	(66)	(74)	(172)
Amortization of net loss	0	0	0
Amortization of prior service cost	27	28	30
Effect of settlement	0	(5)	68
Net pension benefit cost	45	76	167
Other Post-Retirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0
Interest cost	14	23	20
Expected return on plan assets	0	0	0
Amortization of net loss	0	(4)	0
Amortization of prior service cost	0	0	0
Effect of settlement	0	(5)	0
Net pension benefit cost	$ 14	$ 14	$ 20