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Equity-Based Compensation - Performance Share Units ("PSUs") (Details) - Performance Share Units (PSUs) - USD ($) $ / shares in Units, $ 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]
Award vesting period	3 years
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Nonvested beginning of period (in shares)	363,947
Grants (in shares)	254,451
Attainment adjustment (in shares)	361,880
Vested (in shares)	(530,569)
Forfeited (in shares)	(30,736)
Nonvested end of period (in shares)	418,973	363,947
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Beginning nonvested, weighted average grant date fair value (in dollars per share)	$ 38.92
Granted, weighted average grant date fair value (in dollars per share)	59.00	$ 39.91	$ 37.83
Attainment adjustment (in USD per share)	24.40
Vested, weighted average grant date fair value (in dollars per share)	37.84
Forfeited, weighted average grant date fair value (in dollars per share	47.28
Ending nonvested, weighted average grant date fair value (in dollars per share)	$ 50.75	$ 38.92
Vested in period, fair value (less than)	$ 20	$ 9	$ 0
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Potential vesting percentage range of shares	0.00%
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Potential vesting percentage range of shares	200.00%

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Potential Vesting Percentage Range of Shares

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Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Attainment Adjustment in the Period

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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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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.

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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).

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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.

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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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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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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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