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Stock-Based Compensation (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Number of shares authorized | 11,000,000 |  |
Number of shares available for grant | 2,800,000.0 |  |
Allocated share-based compensation expense | $ 36 | $ 36 | $ 14
Tax benefit from compensation expense | 18 | $ 12 | $ 5
Compensation cost not yet recognized | $ 41 |  |
Compensation cost not yet recognized, period for recognition | 1 year 9 months |  |
Performance Based Restricted Stock Units (RSUs) |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Award vesting period | 4 years |  |
Performance Shares | Annual Performance Share Plan |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Measurement period of specific annual performance goals, in years | 1 year |  |
Cliff vesting period following date of grant | 2 years |  |
Performance Shares | Long Term Incentive Plan |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Measurement period of specific annual performance goals, in years | 3 years |  |
Performance Shares | Special Supplemental Equity Awards |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Service based vesting period | 2 years |  |
Minimum | Restricted Stock Units (RSUs) |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Award vesting period | 2 years |  |
Minimum | Performance Based Restricted Stock Units (RSUs) |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Award vesting rights, percentage | 0.00% |  |
Minimum | Performance Shares | Annual Performance Share Plan |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Award vesting rights, percentage | 0.00% |  |
Minimum | Performance Shares | Long Term Incentive Plan |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Award vesting rights, percentage | 0.00% |  |
Maximum | Restricted Stock Units (RSUs) |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Award vesting period | 3 years |  |
Maximum | Performance Based Restricted Stock Units (RSUs) |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Award vesting rights, percentage | 100.00% |  |
Maximum | Performance Shares | Annual Performance Share Plan |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Award vesting rights, percentage | 200.00% |  |
Maximum | Performance Shares | Long Term Incentive Plan |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Award vesting rights, percentage | 200.00% |  |

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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5047-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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