Share-Based Compensation - Nonvested Restricted Shares (Details) $ / shares in Units, $ in Millions	6 Months Ended
Jun. 30, 2019 USD ($) $ / shares shares
Weighted Avg. Grant Date Fair Value
Unrecognized compensation costs \| $	$ 1
Unrecognized compensation costs, not yet recognized	1 year
Restricted Shares
Nonvested Restricted Shares
Nonvested, Beginning Balance (in shares) \| shares	178,550
Granted (in shares) \| shares	34,009
Vested (in shares) \| shares	(172,050)
Forfeited (in shares) \| shares	(3,329)
Nonvested, Ending Balance (in shares) \| shares	37,180
Weighted Avg. Grant Date Fair Value
Nonvested, Beginning Balance (in dollars per share) \| $ / shares	$ 55.46
Granted (in dollars per share) \| $ / shares	34.66
Vested (in dollars per share) \| $ / shares	50.12
Forfeited (in dollars per share) \| $ / shares	62.12
Nonvested, Ending Balance (in dollars per share) \| $ / shares	$ 63.30
Share-Settled RSUs
Nonvested Restricted Shares
Nonvested, Beginning Balance (in shares) \| shares	440,403
Granted (in shares) \| shares	867,137
Vested (in shares) \| shares	(88,590)
Forfeited (in shares) \| shares	(50,113)
Nonvested, Ending Balance (in shares) \| shares	1,168,837
Weighted Avg. Grant Date Fair Value
Nonvested, Beginning Balance (in dollars per share) \| $ / shares	$ 47.99
Granted (in dollars per share) \| $ / shares	34.21
Vested (in dollars per share) \| $ / shares	54.60
Forfeited (in dollars per share) \| $ / shares	41.26
Nonvested, Ending Balance (in dollars per share) \| $ / shares	$ 38.04
Unrecognized compensation costs \| $	$ 54
Unrecognized compensation costs, not yet recognized	2 years
PSUs
Nonvested Restricted Shares
Nonvested, Beginning Balance (in shares) \| shares	227,049
Granted (in shares) \| shares	634,511
Vested (in shares) \| shares	0
Forfeited (in shares) \| shares	(43,527)
Nonvested, Ending Balance (in shares) \| shares	818,033
Weighted Avg. Grant Date Fair Value
Nonvested, Beginning Balance (in dollars per share) \| $ / shares	$ 49.18
Granted (in dollars per share) \| $ / shares	24.77
Vested (in dollars per share) \| $ / shares	0
Forfeited (in dollars per share) \| $ / shares	43.61
Nonvested, Ending Balance (in dollars per share) \| $ / shares	$ 34.25

Unrecognized cost of unvested share-based compensation awards.

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-Publisher FASB
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-Section 50
-Paragraph 2
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- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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- Definition

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

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- Definition

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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- Definition

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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- Definition

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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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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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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- Definition

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

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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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