Employee Benefit Plans - Components of Net Period Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 101	$ 201	$ 185	$ 401
Interest cost	685	743	1,372	1,489
Expected return on plan assets	(898)	(776)	(1,798)	(1,552)
Amortization of net loss	491	446	980	1,122
Amortization of prior service cost	7	53	18	106
Effect of settlement		163		395
Net pension benefit cost	386	830	757	1,961
Pension | Retirement Income Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	8	17	15	33
Interest cost	354	395	709	791
Expected return on plan assets	(512)	(439)	(1,024)	(878)
Amortization of net loss	312	301	623	774
Amortization of prior service cost	0	0	0	0
Effect of settlement		104		277
Net pension benefit cost	162	378	323	997
Pension | National Service-Related Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	20	126	39	252
Interest cost	300	314	600	628
Expected return on plan assets	(342)	(294)	(685)	(588)
Amortization of net loss	179	145	357	348
Amortization of prior service cost	0	53	0	106
Effect of settlement		59		118
Net pension benefit cost	157	403	311	864
Pension | Superannuation
Defined Benefit Plan Disclosure [Line Items]
Service cost	73	58	131	116
Interest cost	26	29	53	59
Expected return on plan assets	(44)	(43)	(89)	(86)
Amortization of net loss	0	0	0	0
Amortization of prior service cost	7	0	18	0
Effect of settlement		0		0
Net pension benefit cost	62	44	113	89
Other Post-Retirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	0
Interest cost	5	5	10	11
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
Effect of settlement		0		0
Net pension benefit cost	$ 5	$ 5	$ 10	$ 11