Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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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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Number of shares authorized for issuance under share-based payment arrangement.

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Percentage of vesting of award under share-based payment arrangement.

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Stock-Based Compensation (Narrative) (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for awards to employees and outside directors (in shares)	10
RSU awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total grant date fair value of shares vested	$ 43	$ 49	$ 43
Unrecognized compensation cost	$ 31
Unrecognized compensation cost, period of recognition, years	23 months
RSU awards \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Performance awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Total grant date fair value of shares vested	$ 36	$ 23
Unrecognized compensation cost	$ 23
Unrecognized compensation cost, period of recognition, years	21 months
Risk free interest rate	1.40%
Expected volatility rate	13.60%
Performance awards \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percent	200.00%