Post Retirement Benefits - Effect of changing the health care cost trend rate by one-percentage point for each future year (Details) - Post retirement health and life insurance benefit plan	12 Months Ended
Dec. 31, 2020 USD ($)
Postretirement benefits:
Effect on total of service and interest cost components, 1 - percentage point increase	$ 34,000
Effect on total of service and interest cost components, 1 - percentage point decrease	(29,000)
Effect on post retirement benefit obligation, 1 - percentage point increase	1,081,000
Effect on post retirement benefit obligation, 1 - percentage point decrease	$ (924,000)

Amount of decrease in accumulated postretirement benefit obligation from one-percentage-point decrease in assumed health care cost trend rate.

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (m)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

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Amount of decrease in service and interest cost components of net periodic postretirement benefit cost from one-percentage-point decrease in assumed health care cost trend rate.

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Amount of increase in accumulated postretirement benefit obligation from one-percentage-point increase in assumed health care cost trend rate.

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Amount of increase in service and interest cost components of net periodic postretirement benefit cost from one-percentage-point increase in assumed health care cost trend rate.

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