Pension Benefits - Net Periodic Benefit and Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net periodic benefit cost:
Service cost	$ 1,200	$ 1,300	$ 1,300
Interest cost	6,431	7,993	7,386
Expected return on plan assets	(13,119)	(13,296)	(13,737)
Amortization of net actuarial loss	985	795	721
Total net periodic benefit	(4,503)	(3,208)	(4,330)
Other comprehensive income:
Current year actuarial gain	(9,345)	(9,006)	167
Recognized net actuarial loss	985	795	721
Total other comprehensive gain	$ (8,360)	$ (8,211)	$ 888