Stock Compensation Plans - Narrative (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Payment Arrangement, Option, Exercise Price Range, Exercisable, Weighted Average Exercise Price	$ 38.26
Share Based Compensation, Stock Options Award Exercise Price Premium Percentage	115.00%
Weighted average grant date fair value of stock options (in dollars per share)	$ 7.85	$ 8.07
Contractual term of stock options granted	7 years	7 years
Risk-free interest rate period of U.S. Treasury yield	7 years
Unrecognized compensation cost related to unvested awards	$ 63
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)	0.5	0.3
Discounted price per share of stocks purchased by employees (in dollar per share)	$ 15.05	$ 20.24
Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of stock options grated	3 years	4 years
Weighted average period to recognized compensation cost related to unvested awards	1 year 4 months 24 days
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost related to unvested awards	$ 118
Weighted average period to recognized compensation cost related to unvested awards	1 year 1 month 6 days

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

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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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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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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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Weighted average exercise price as of the balance sheet date for those equity-based payment arrangements exercisable and outstanding.

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

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