Pension Benefits - Benefit Plan Obligation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in benefit obligation:
Beginning balance	$ 221,458	$ 201,752	$ 218,746
Interest cost	6,431	7,993	7,386
Actuarial loss (gain)	18,243	25,632	(16,324)
Benefits paid	(9,006)	(13,919)	(8,056)
Benefit obligation at end of period	237,126	221,458	201,752
Change in fair value of plan assets:
Beginning balance	205,463	184,758	190,534
Actual return on assets	21,987	29,815	(2,497)
Employer contributions	1,500	6,000	6,000
Administrative fees	(9,006)	(13,918)	(1,223)
Benefits paid	(1,171)	(1,192)	(8,056)
Fair value of plan assets at end of period	$ 218,773	$ 205,463	$ 184,758