Postretirement Benefits - Other Comprehensive Income (Details) $ in Thousands	12 Months Ended
Dec. 31, 2019 USD ($) participant	Dec. 31, 2018 USD ($) participant
Amounts recognized in other comprehensive income for pension plans:
Net actuarial loss (gain) arising during the year	$ 3,626	$ (34,290)
Amortization of net actuarial loss	(5,113)	(12,809)
Amortization of prior service cost	(1,394)	(1,353)
Amendments	452	3,007
Total recognized in other comprehensive income	$ (2,429)	$ (45,445)
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	5

The number of new plan participants for which amendment charges were recognized.

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Amount, before tax, of other comprehensive income from plan amendments, that has not been recognized in net periodic benefit cost.

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Amount, before tax, of reclassification adjustment from accumulated other comprehensive (income) loss for prior service cost (credit) of defined benefit plan.

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Amount, before tax, after reclassification adjustment, of (increase) decrease in accumulated other comprehensive income for defined benefit plan.

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Amount, before tax, of gain (loss) for (increase) decrease in value of benefit obligation for change in actuarial assumptions and increase (decrease) in value of plan assets from experience different from that assumed of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

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Amount, before tax, of reclassification adjustment from accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan.

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