Employee Benefit Plans - Components of Net Period Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 16	$ 14	$ 32	$ 28
Interest cost	478	605	956	1,209
Expected return on plan assets	(1,042)	(883)	(2,084)	(1,765)
Amortization of net loss	381	405	762	811
Amortization of prior service cost	15	7	15	15
Net pension benefit cost	(152)	148	(319)	298
Pension | Retirement Income Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	0
Interest cost	237	316	474	631
Expected return on plan assets	(596)	(501)	(1,192)	(1,001)
Amortization of net loss	218	254	436	508
Amortization of prior service cost	0	0	0	0
Net pension benefit cost	(141)	69	(282)	138
Pension | National Service-Related Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	0
Interest cost	234	280	468	559
Expected return on plan assets	(427)	(367)	(854)	(733)
Amortization of net loss	163	151	326	303
Amortization of prior service cost	0	0	0	0
Net pension benefit cost	(30)	64	(60)	129
Pension | Superannuation
Defined Benefit Plan Disclosure [Line Items]
Service cost	16	14	32	28
Interest cost	5	6	10	12
Expected return on plan assets	(19)	(15)	(38)	(31)
Amortization of net loss	0	0	0	0
Amortization of prior service cost	15	7	15	15
Net pension benefit cost	17	12	19	24
Other Post-Retirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	0
Interest cost	2	3	4	7
Expected return on plan assets	0	0	0	0
Amortization of net loss	0	0	0	0
Amortization of prior service cost	0	0	0	0
Net pension benefit cost	$ 2	$ 3	$ 4	$ 7