Employee Benefit Plans - Components of Net Period Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 246	$ 722	$ 755
Interest cost	2,741	2,981	3,184
Expected return on plan assets	(3,588)	(3,106)	(3,479)
Amortization of net loss	1,959	2,703	2,867
Amortization of prior service cost	30	221	212
Effect of settlement	68	63	726
Net pension benefit cost	1,456	3,584	4,265
Pension | Retirement Income Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	31	65	108
Interest cost	1,418	1,583	1,767
Expected return on plan assets	(2,047)	(1,757)	(2,072)
Amortization of net loss	1,244	1,889	2,125
Amortization of prior service cost	0	0	0
Effect of settlement	0	0	737
Net pension benefit cost	646	1,780	2,665
Pension | National Service-Related Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	78	504	516
Interest cost	1,199	1,256	1,288
Expected return on plan assets	(1,369)	(1,175)	(1,244)
Amortization of net loss	715	815	745
Amortization of prior service cost	0	212	212
Effect of settlement	0	0	0
Net pension benefit cost	623	1,612	1,517
Pension | Superannuation
Defined Benefit Plan Disclosure [Line Items]
Service cost	137	153	131
Interest cost	104	120	104
Expected return on plan assets	(172)	(174)	(163)
Amortization of net loss	0	0	0
Amortization of prior service cost	30	9	0
Effect of settlement	68	67	0
Net pension benefit cost	167	175	72
Other Post-Retirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0
Interest cost	20	22	25
Expected return on plan assets	0	0	0
Amortization of net loss	0	(1)	(3)
Amortization of prior service cost	0	0	0
Effect of settlement	0	(4)	(11)
Net pension benefit cost	$ 20	$ 17	$ 11