Postretirement Benefits - Other Comprehensive Income (Details) $ in Thousands	12 Months Ended
Dec. 31, 2018 USD ($) participant	Dec. 31, 2017 USD ($) participant
Amounts recognized in other comprehensive income for pension plans:
Net actuarial (gain) loss arising during the year	$ (34,290)	$ 15,588
Amortization of net actuarial loss	(12,809)	(9,301)
Amortization of prior service cost	(1,353)	(871)
Amendments	3,007	5,050
Impact due to settlement	0	(302)
Total recognized in other comprehensive income	(45,445)	10,164
Impact of amendments on pension obligation	$ 3,007	5,050
Employee pension plan \| Qualified Plan \| Funded Plan
Amounts recognized in other comprehensive income for pension plans:
Impact of amendments on pension obligation		$ 3,600
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	5	2
Impact of amendments on pension obligation		$ 300
Impact of amendments on prior service cost		$ 1,200
Number of former plan participants resulting in charges recognized as settlements \| participant		2

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The number of former plan participants for which settlement 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 of increase (decrease) in benefit obligation of defined benefit plan from change in terms of existing plan or initiation of new plan.

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Amount, before tax, of cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation 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 prior service cost (credit) of defined benefit plan.

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Amount, before tax, of increase (decrease) to other comprehensive income from settlement and curtailment gain (loss) of defined benefit plan.

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