Stock-Based Compensation - Narrative (Details) - USD ($)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation expense for stock-based compensation plans	$ 99,000,000	$ 104,000,000	$ 102,000,000
Total income tax benefit recognized for stock-based compensation plans	$ 23,000,000	$ 39,000,000	$ 38,000,000
Common Shares available for future grant under compensation plans (in shares)	24,147,422
Maximum percentage of outstanding common stock that may be granted as options	6.00%
Rolling period in which a certain percentage of common stock cannot be granted as options	3 years
Exercise rate of stock options granted to employees	25.00%
Vesting period for compensation cost	4 years
Common shares, shares issued (in shares)	1,256,702,081	1,256,702,081
Weighted-average grant-date fair value of options (in dollars per share)	$ 5.12	$ 4.6	$ 2.14
Number of options, exercised (in shares)	1,960,444
Total intrinsic value of exercised options	$ 21,000,000	$ 31,000,000	$ 12,000,000
Cash received from options exercised	20,000,000	25,000,000	32,000,000
Actual tax benefit realized for tax deductions from options exercised	$ 1,000,000	$ 4,000,000	$ 2,000,000
Mandatory deferred incentive awards, vesting rate (as a percent)	25.00%
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares, shares issued (in shares)	3,000,000
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options expiration years	10 years
Number of options, exercised (in shares)	1,960,444	3,755,177	2,849,010
Unrecognized compensation cost related to nonvested options expected to vest	$ 2,000,000
Weighted-average period	2 years
Stock Options [Member] \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period for compensation cost	1 year
Stock Options [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options expiration years	10 years
Deferred Cash Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage	25.00%
Restricted Stock Unit [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage	25.00%
Equity Based Performance Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Cash Performance Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Deferred Compensation Plans and Other Restricted Stock Award [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average grant-date fair value granted (in dollars per share)	$ 20.77	$ 18.55	$ 11.46
Deferred Compensation Plans [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost related to nonvested options expected to vest	$ 21,000,000
Weighted-average period	2 years 10 months 24 days
Fair value of units/shares vested	$ 22,000,000	$ 21,000,000	$ 16,000,000
Long-Term Incentive Compensation Program [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost related to nonvested options expected to vest	$ 73,000,000
Weighted-average period	2 years 4 months 24 days
Number of performance units vested (in shares)	1,070,112	887,489
Fair value of units/shares vested	$ 93,000,000	$ 76,000,000	$ 57,000,000
Weighted-average grant-date fair value granted (in dollars per share)	$ 19.28	$ 19.82	$ 10.49
Long-Term Incentive Compensation Program [Member] \| Performance Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of units/shares vested	$ 19,000,000	$ 14,000,000
Discounted Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee discount on the purchase of stock through Discounted Stock Purchase Plan	10.00%
Purchases are limited to any month	$ 10,000
Purchases are limited to any calendar year	$ 50,000
Issuance of common shares (in shares)	327,435	257,738	310,604
Weighted-average cost of common shares issued under the plan (in dollars per share)	17.48	16.61	11.04

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Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.

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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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