Stock Compensation Plans - Stock-based Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units	$ 23	$ 25	$ 55	$ 67
Stock options	6	2	16	6
Employee stock purchase plan	2	1	5	3
Stock-based compensation expense	31	28	76	76
Tax Benefit	(4)	(5)	(9)	(12)
Stock-based compensation expense (net of tax)	$ 27	$ 23	$ 67	$ 64

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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Amount, after tax, of expense for award under share-based payment arrangement.

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Amount of tax benefit for recognition of expense of award under share-based payment arrangement.

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The amount of plan compensation cost recognized during the period.

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Amount of noncash expense for award of restricted stock or unit under share-based payment arrangement.

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Amount of noncash expense for option under share-based payment arrangement.

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