Pensions and Other Post-retirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,264	$ 3,012	$ 9,748	$ 9,035
Interest cost	2,941	3,726	8,575	11,178
Expected return on plan assets	(9,532)	(8,503)	(27,826)	(25,509)
Amortization of prior service cost (credit)	66	46	198	138
Recognized net actuarial losses	4,421	3,935	13,263	11,805
Settlements			(1,879)	0
Net periodic benefit (income) cost	1,160	2,216	2,079	6,647
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	99	99	297	297
Interest cost	121	179	353	537
Expected return on plan assets	0	0	0	0
Amortization of prior service cost (credit)	(90)	(98)	(270)	(294)
Recognized net actuarial losses	399	286	1,197	858
Settlements			0	0
Net periodic benefit (income) cost	$ 529	$ 466	$ 1,577	$ 1,398