Pensions and Other Post-retirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,736	$ 2,891	$ 5,159	$ 5,782
Interest cost	4,721	4,219	9,426	8,438
Expected return on plan assets	(9,658)	(9,096)	(19,311)	(18,193)
Amortization of prior service cost (credit)	112	(6)	108	(11)
Recognized net actuarial losses	2,534	3,453	5,111	6,907
Settlements	2,363	27	2,363	53
Net periodic benefit cost	2,808	1,488	2,856	2,976
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	89	83	178	184
Interest cost	249	175	498	396
Expected return on plan assets	0	0	0	0
Amortization of prior service cost (credit)	(101)	(125)	(202)	(202)
Recognized net actuarial losses	217	351	434	376
Settlements	0	141	0	0
Net periodic benefit cost	$ 454	$ 625	$ 908	$ 754