Pensions and Other Post-retirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,012	$ 2,423
Interest cost	3,726	4,705
Expected return on plan assets	(8,503)	(9,653)
Amortization of prior service cost (credit)	46	(4)
Recognized net actuarial losses	3,935	2,577
Net periodic benefit cost	2,216	48
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	99	89
Interest cost	179	249
Expected return on plan assets	0	0
Amortization of prior service cost (credit)	(98)	(101)
Recognized net actuarial losses	286	217
Net periodic benefit cost	$ 466	$ 454