Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee.

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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.

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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.

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Benefit Plans - Weighted average target allocation and actual percentage (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Private equity funds
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Actual percentage of plan assets	0.00%	1.00%
Mutual funds
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Target asset allocation	1.00%	0.00%
Pension benefits
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Actual percentage of plan assets	100.00%	100.00%
Fair value of plan assets	$ 6,987	$ 6,192	$ 5,299
Pension benefits \| Equity securities
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Actual percentage of plan assets	50.00%	50.00%
Pension benefits \| Equity securities \| Minimum
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Target asset allocation	43.00%
Pension benefits \| Equity securities \| Maximum
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Target asset allocation	62.00%
Pension benefits \| Fixed income securities
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Actual percentage of plan assets	38.00%	38.00%
Pension benefits \| Fixed income securities \| Minimum
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Target asset allocation	33.00%
Pension benefits \| Fixed income securities \| Maximum
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Target asset allocation	45.00%
Pension benefits \| Limited partnership interests
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Actual percentage of plan assets	10.00%	10.00%
Pension benefits \| Limited partnership interests \| Minimum
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Target asset allocation	0.00%
Pension benefits \| Limited partnership interests \| Maximum
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Target asset allocation	15.00%
Pension benefits \| Short-term investments and other
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Target asset allocation	0.00%
Actual percentage of plan assets	2.00%	2.00%
Pension benefits \| Securities pledged as collateral
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Fair value of plan assets	$ 101	$ 258