Employee Benefit Plans - Change in Postretirement Benefit Obligation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in Benefit Obligation
Benefit obligation at beginning of year	$ 34,438	$ 29,777	$ 31,050
Service cost	1,493	1,533	1,776
Interest cost	974	1,283	1,172
Actuarial (gain) loss	(2,119)	3,279	(3,165)
Benefits paid	(2,042)	(1,434)	(1,056)
Benefit obligation at end of year	32,744	34,438	29,777
Change in Plan Assets
Fair value of plan assets at end of year	0	0	0
Funded status at end of year	$ (32,744)	$ (34,438)	$ (29,777)