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Retirement benefits - Asset Allocation Target and Actual Percentage (Details)	Dec. 31, 2020	Dec. 31, 2019
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	100.00%	100.00%
Asset category, Target
Target	100.00%
Pension benefits | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	72.00%	71.00%
Asset category, Target
Target	70.00%
Pension benefits | Equity securities | Minimum
Asset category, Target
Target	65.00%
Pension benefits | Equity securities | Maximum
Asset category, Target
Target	75.00%
Pension benefits | Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	28.00%	29.00%
Asset category, Target
Target	30.00%
Pension benefits | Fixed income securities | Minimum
Asset category, Target
Target	25.00%
Pension benefits | Fixed income securities | Maximum
Asset category, Target
Target	35.00%
Other benefits
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	100.00%	100.00%
Asset category, Target
Target	100.00%
Other benefits | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	73.00%	71.00%
Asset category, Target
Target	70.00%
Other benefits | Equity securities | Minimum
Asset category, Target
Target	65.00%
Other benefits | Equity securities | Maximum
Asset category, Target
Target	75.00%
Other benefits | Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	27.00%	29.00%
Asset category, Target
Target	30.00%
Other benefits | Fixed income securities | Minimum
Asset category, Target
Target	25.00%
Other benefits | Fixed income securities | Maximum
Asset category, Target
Target	35.00%

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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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