Pensions and Other Post-retirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,242	$ 3,011	$ 6,484	$ 6,022
Interest cost	2,817	3,726	5,634	7,452
Expected return on plan assets	(9,147)	(8,503)	(18,294)	(17,006)
Amortization of prior service cost (credit)	66	46	132	92
Recognized net actuarial losses	4,421	3,935	8,842	7,870
Settlements			(1,879)	0
Net periodic benefit (income) cost	1,399	2,215	919	4,430
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	99	99	198	198
Interest cost	116	179	232	358
Expected return on plan assets	0	0	0	0
Amortization of prior service cost (credit)	(90)	(98)	(180)	(196)
Recognized net actuarial losses	399	286	798	572
Settlements			0	0
Net periodic benefit (income) cost	$ 524	$ 466	$ 1,048	$ 932