Stock-Based Compensation- Restricted Stock and Restricted Stock Units Activity (Details) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
Mar. 31, 2017 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-vested shares outstanding, Beginning balance	2,433
Granted	591
Forfeited	(19)
Vested	(37)
Non-vested shares outstanding, Ending balance	2,968
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized \| $	$ 76
Service - Based Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-vested shares outstanding, Beginning balance	1,574
Granted	268
Forfeited	(19)
Vested	(37)
Non-vested shares outstanding, Ending balance	1,786
Non-vested shares outstanding, Weighted Average Grant Date Fair Value per Share, Beginning balance \| $ / shares	$ 35.56
Granted, Weighted Average Grant Date Fair Value per Share \| $ / shares	39.56
Forfeited, Weighted Average Grant Date Fair Value per Share \| $ / shares	18.70
Vested, Weighted Average Grant Date Fair Value per Share \| $ / shares	25.38
Non-vested shares outstanding, Weighted Average Grant Date Fair Value per Share, Ending balance \| $ / shares	$ 36.41
Performance Market Based Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-vested shares outstanding, Beginning balance	859
Granted	323
Forfeited	0
Vested	0
Non-vested shares outstanding, Ending balance	1,182
Non-vested shares outstanding, Weighted Average Grant Date Fair Value per Share, Beginning balance \| $ / shares	$ 26.28
Granted, Weighted Average Grant Date Fair Value per Share \| $ / shares	50.22
Forfeited, Weighted Average Grant Date Fair Value per Share \| $ / shares	0.00
Vested, Weighted Average Grant Date Fair Value per Share \| $ / shares	0.00
Non-vested shares outstanding, Weighted Average Grant Date Fair Value per Share, Ending balance \| $ / shares	$ 32.81
Cash-Settled Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-vested shares outstanding, Beginning balance	460
Granted	235
Forfeited	(12)
Vested	(26)
Non-vested shares outstanding, Ending balance	657
Minimum [Member] \| Performance Market Based Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage	0.00%
Maximum [Member] \| Performance Market Based Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage	200.00%
Retirement Eligible [Member] [Member] \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage	50.00%
Retirement Eligible [Member] [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage	100.00%

Unrecognized cost of unvested share-based compensation awards.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Percentage of vesting of share-based compensation awards.

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