Employee Benefit Plans - Components of Net Period Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 14	$ 18	$ 28	$ 34
Interest cost	605	727	1,209	1,454
Expected return on plan assets	(883)	(753)	(1,765)	(1,505)
Amortization of net loss	405	517	811	1,034
Amortization of prior service cost	7	9	15	17
Net pension benefit cost	148	518	298	1,034
Other Post-Retirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	0
Interest cost	3	6	7	12
Expected return on plan assets	0	0	0	0
Amortization of net loss	0	(1)	0	(2)
Amortization of prior service cost	0	0	0	0
Net pension benefit cost	3	5	7	10
Retirement Income Plan | Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	0
Interest cost	316	397	631	795
Expected return on plan assets	(501)	(440)	(1,001)	(880)
Amortization of net loss	254	377	508	754
Amortization of prior service cost	0	0	0	0
Net pension benefit cost	69	334	138	669
National Service-Related Pension Plan | Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	0
Interest cost	280	311	559	622
Expected return on plan assets	(367)	(294)	(733)	(588)
Amortization of net loss	151	141	303	282
Amortization of prior service cost	0	0	0	0
Net pension benefit cost	64	158	129	316
Superannuation | Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	14	18	28	34
Interest cost	6	13	12	25
Expected return on plan assets	(15)	(19)	(31)	(37)
Amortization of net loss	0	0	0	0
Amortization of prior service cost	7	9	15	17
Net pension benefit cost	$ 12	$ 21	$ 24	$ 39