Pensions and Other Post-retirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,242	$ 3,012
Interest cost	2,817	3,726
Expected return on plan assets	(9,147)	(8,503)
Amortization of prior service cost (credit)	66	46
Recognized net actuarial losses	4,421	3,935
Settlements	1,879	0
Net periodic benefit cost	(480)	2,216
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	99	99
Interest cost	116	179
Expected return on plan assets	0	0
Amortization of prior service cost (credit)	(90)	(98)
Recognized net actuarial losses	399	286
Settlements	0	0
Net periodic benefit cost	$ 524	$ 466