Retirement Benefits - Schedule of Assumptions Used to Determine Periodic Benefit Expense (Income) (Details)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected long-term rate of return on assets (percentage)	7.00%	7.00%	7.00%
Medical and Life Insurance Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate (percentage)	3.19%	4.09%	3.37%
Ultimate healthcare trend rate (percentage)	4.50%	4.50%	5.00%
Initial healthcare trend rate (pre 65/post 65) (percentage)	5.50%	6.00%	6.50%
Qualified Plan \| Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate (percentage)	3.28%	4.27%	3.59%
Nonqualified Plan \| Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate (percentage)	3.30%	4.27%	3.62%

Defined Benefit Plan, Assumption Used Calculating Net Periodic Benefit Cost, Ultimate Health Care Cost Trend Rate

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- Definition

Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost of defined benefit plan.

+ References

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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

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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- Definition

Assumed rate, for next fiscal year, based on annual change in cost of health care cost benefits used to measure expected cost of benefits covered by defined benefit postretirement plan. Factors include, but are not limited to, estimate of health care inflation, change in health care utilization or delivery pattern, technological advances, and change in health status of participant. Excludes factors for change in composition of plan population by age and dependency status.

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