Employee Benefit Plans - Components of Net Period Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 18	$ 101	$ 34	$ 185
Interest cost	727	685	1,454	1,372
Expected return on plan assets	(753)	(898)	(1,505)	(1,798)
Amortization of net loss	517	491	1,034	980
Amortization of prior service cost	9	7	17	18
Net pension benefit cost	518	386	1,034	757
Other Post-Retirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	0
Interest cost	6	5	12	10
Expected return on plan assets	0	0	0	0
Amortization of net loss	(1)	0	(2)	0
Amortization of prior service cost	0	0	0	0
Net pension benefit cost	5	5	10	10
Retirement Income Plan | Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	8	0	15
Interest cost	397	354	795	709
Expected return on plan assets	(440)	(512)	(880)	(1,024)
Amortization of net loss	377	312	754	623
Amortization of prior service cost	0	0	0	0
Net pension benefit cost	334	162	669	323
National Service-Related Pension Plan | Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	20	0	39
Interest cost	311	300	622	600
Expected return on plan assets	(294)	(342)	(588)	(685)
Amortization of net loss	141	179	282	357
Amortization of prior service cost	0	0	0	0
Net pension benefit cost	158	157	316	311
Superannuation | Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	18	73	34	131
Interest cost	13	26	25	53
Expected return on plan assets	(19)	(44)	(37)	(89)
Amortization of net loss	0	0	0	0
Amortization of prior service cost	9	7	17	18
Net pension benefit cost	$ 21	$ 62	$ 39	$ 113