Stock Compensation Plans - Narrative (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant date fair value of stock options (in dollars per share)	$ 8.07	$ 9.80
Contractual term of stock options granted	7 years	7 years
Unrecognized compensation cost related to unvested awards	$ 33
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate period of U.S. Treasury yield	3 years
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate period of U.S. Treasury yield	5 years
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Discount stock purchase price percentage	15.00%
Look-back feature period of discount stock purchase price	3 months
Minimum contribution by participant percentage	1.00%
Maximum contribution by participant percentage	10.00%
Stocks purchased by employees during the period (in shares)	0.3	0.2
Discounted price per share of stocks purchased by employees (in dollar per share)	$ 20.24	$ 29.62
Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of stock options grated	4 years	4 years
Weighted average period to recognized compensation cost related to unvested awards	1 year 9 months 18 days
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost related to unvested awards	$ 121
Weighted average period to recognized compensation cost related to unvested awards	1 year

Share-based Compensation Arrangement by Share-based Payment Award, Discount from Market Price, Look-Back Period

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- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate, Period of U.S. Treasury Yield

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Share-based Compensation Arrangement by Share-based Payment Award, Minimum Employee Subscription Rate

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Stock Issued During Period, Price Per Share, Employee Stock Purchase Plans

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Unrecognized cost of unvested share-based compensation awards.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Discount rate from fair value on purchase date that participants pay for 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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The highest percentage of annual salary that an employee is permitted to utilize with respect to the plan.

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- Definition

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of shares issued during the period as a result of an employee stock purchase plan.

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