Stock Compensation Plans - Stock-based Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units	$ 25	$ 13	$ 67	$ 50
Stock options	2	1	6	2
Employee stock purchase plan	1	1	3	3
Stock-based compensation expense	28	15	76	55
Tax benefit	(5)	0	(12)	(7)
Total stock-based compensation expense (net of tax)	$ 23	$ 15	$ 64	$ 48

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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The amount of plan compensation cost recognized during the period.

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

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