Pension Benefits - Benefit Plan Obligation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Change in benefit obligation:
Beginning balance	$ 201,752	$ 218,746	$ 216,571
Interest cost	7,993	7,386	8,283
Actuarial loss (gain)	25,632	(16,324)	11,402
Benefits paid	(13,919)	(8,056)	(17,510)
Benefit obligation at end of period	221,458	201,752	218,746
Change in fair value of plan assets:
Beginning balance	184,758	190,534	178,375
Actual return on assets	29,815	(2,497)	25,240
Employer contributions	6,000	6,000	6,000
Administrative fees	(13,918)	(1,223)	(1,571)
Benefits paid	(1,192)	(8,056)	(17,510)
Fair value of plan assets at end of period	$ 205,463	$ 184,758	$ 190,534