Stock Compensation Plans - Stock-based Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units	$ 22	$ 24	$ 42	$ 37
Stock options	2	1	4	1
Employee stock purchase plan	1	1	2	2
Stock-based compensation expense	25	26	48	40
Tax benefit	(4)	(4)	(7)	(7)
Total stock-based compensation expense (net of tax)	$ 21	$ 22	$ 41	$ 33

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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