Employee Benefit Plans - Components of Net Period Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 85	$ 200
Interest cost	686	746
Expected return on plan assets	(899)	(776)
Amortization of net loss	490	676
Amortization of prior service cost	8	53
Effect of settlement		(232)
Net pension benefit cost	370	1,131
Pension | Retirement Income Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	8	16
Interest cost	354	396
Expected return on plan assets	(512)	(439)
Amortization of net loss	311	472
Amortization of prior service cost	0	0
Effect of settlement		(173)
Net pension benefit cost	161	618
Pension | National Service-Related Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	19	126
Interest cost	300	314
Expected return on plan assets	(342)	(294)
Amortization of net loss	179	204
Amortization of prior service cost	0	53
Effect of settlement		(59)
Net pension benefit cost	156	462
Pension | Superannuation
Defined Benefit Plan Disclosure [Line Items]
Service cost	58	58
Interest cost	27	30
Expected return on plan assets	(45)	(43)
Amortization of net loss	0	0
Amortization of prior service cost	8	0
Effect of settlement		0
Net pension benefit cost	48	45
Other Post-Retirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0
Interest cost	5	6
Expected return on plan assets	0	0
Amortization of net loss	0	0
Amortization of prior service cost	0	0
Effect of settlement		0
Net pension benefit cost	$ 5	$ 6