Postretirement Benefits - Other Comprehensive Income (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($) participant
Amounts recognized in other comprehensive income for pension plans:
Net actuarial (gain) loss arising during the year	$ (11,832)	$ 3,626
Amortization of net actuarial loss	(12,125)	(5,113)
Amortization of prior service cost	(1,343)	(1,394)
Amendments	0	452
Total recognized in other comprehensive income	$ (25,300)	$ (2,429)
Supplemental employee retirement plan (SERP) | Nonqualified Plan | Unfunded plan
Amounts recognized in other comprehensive income for pension plans:
Number of new plan participants resulting in charges recognized as amendments | participant		1