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Retirement Plans - Rate and standard deviation assumptions (Details) - Non-contributory defined benefit pension plan	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan
Rate of Return Assumption (as a percent)	7.75%	7.75%
Cash equivalents
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan
Actual asset allocation (as a percent)	2.00%
Cash equivalents | Maximum
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan
Allowable allocation percentages			35.00%
Cash equivalents | Minimum
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan
Allowable allocation percentages			0.00%
Equities | Maximum
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan
Allowable allocation percentages			65.00%
Equities | Minimum
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan
Allowable allocation percentages			55.00%
Fixed income
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan
Actual asset allocation (as a percent)	98.00%
Fixed income | Maximum
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan
Allowable allocation percentages			40.00%
Fixed income | Minimum
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan
Allowable allocation percentages			20.00%

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- References

No definition available.

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- Definition

Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (k)(3)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2919-114920

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- Definition

Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(i)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

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Percentage of actual investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(i)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

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