Pensions and Other Post-retirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,012	$ 2,579	$ 9,035	$ 7,738
Interest cost	3,726	4,712	11,178	14,138
Expected return on plan assets	(8,503)	(9,655)	(25,509)	(28,966)
Amortization of prior service cost (credit)	46	54	138	162
Recognized net actuarial losses	3,935	2,555	11,805	7,666
Settlements			0	(2,363)
Net periodic benefit cost	2,216	245	6,647	3,101
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	99	88	297	266
Interest cost	179	249	537	747
Expected return on plan assets	0	0	0	0
Amortization of prior service cost (credit)	(98)	(101)	(294)	(303)
Recognized net actuarial losses	286	217	858	651
Settlements			0	0
Net periodic benefit cost	$ 466	$ 453	$ 1,398	$ 1,361