Pension Benefits - Net Periodic Benefit and Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net periodic benefit cost:
Service cost	$ 1,300	$ 1,300	$ 1,700
Interest cost	7,993	7,386	8,283
Expected return on plan assets	(13,296)	(13,737)	(12,831)
Amortization of net actuarial loss	795	721	865
Total net periodic benefit	(3,208)	(4,330)	(1,983)
Other comprehensive income:
Current year actuarial gain	(9,006)	167	1,136
Recognized net actuarial loss	795	721	865
Total other comprehensive gain	$ (8,211)	$ 888	$ 2,001