Common Stock - Nonvested Restricted Shares (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2018	Sep. 30, 2017
Restricted Shares
Nonvested Restricted Shares
Nonvested, Beginning Balance (in shares)	192,831	254,174	410,251	410,251
Granted (in shares)	0	1,573	17,440
Vested (in shares)	(6,443)	(60,434)	(168,409)
Forfeited (in shares)	(1,210)	(2,482)	(5,108)
Nonvested, Ending Balance (in shares)	185,178	192,831	254,174	185,178
Weighted Avg. Grant Date Fair Value
Nonvested, Beginning Balance (in dollars per share)	$ 53.14	$ 52.23	$ 49.95	$ 49.95
Granted (in dollars per share)	0.00	47.97	55.05
Vested (in dollars per share)	53.31	49.89	47.08
Forfeited (in dollars per share)	62.79	57.15	48.68
Nonvested, Ending Balance (in dollars per share)	$ 53.07	$ 53.14	$ 52.23	$ 53.07
Unrecognized compensation costs	$ 4			$ 4
Unrecognized compensation costs, not yet recognized				1 year 2 months 12 days
Total fair value of restricted shares vested				$ 11	$ 14
Share-Settled RSUs
Nonvested Restricted Shares
Nonvested, Beginning Balance (in shares)	260,697	251,895	0	0
Granted (in shares)	14,903	16,995	253,257
Vested (in shares)	0	(192)	0
Forfeited (in shares)	(6,408)	(8,001)	(1,362)
Nonvested, Ending Balance (in shares)	269,192	260,697	251,895	269,192
Weighted Avg. Grant Date Fair Value
Nonvested, Beginning Balance (in dollars per share)	$ 54.51	$ 55.02	$ 0.00	$ 0.00
Granted (in dollars per share)	42.22	47.17	55.02
Vested (in dollars per share)	0.00	55.04	0.00
Forfeited (in dollars per share)	55.04	55.04	55.04
Nonvested, Ending Balance (in dollars per share)	$ 57.51	$ 54.51	$ 55.02	$ 57.51
Unrecognized compensation costs	$ 10			$ 10
Unrecognized compensation costs, not yet recognized				2 years 4 months 24 days
PSUs
Nonvested Restricted Shares
Nonvested, Beginning Balance (in shares)	236,254	214,348	0	0
Granted (in shares)	8,623	25,957	214,348
Vested (in shares)	0	0	0
Forfeited (in shares)	(5,882)	(4,051)	0
Nonvested, Ending Balance (in shares)	238,995	236,254	214,348	238,995
Weighted Avg. Grant Date Fair Value
Nonvested, Beginning Balance (in dollars per share)	$ 49.28	$ 50.75	$ 0.00	$ 0.00
Granted (in dollars per share)	32.24	35.64	50.75
Vested (in dollars per share)	0.00	0.00	0.00
Forfeited (in dollars per share)	50.75	50.75	0.00
Nonvested, Ending Balance (in dollars per share)	$ 48.68	$ 49.28	$ 50.75	$ 48.68
Unrecognized compensation costs	$ 8			$ 8
Unrecognized compensation costs, not yet recognized				2 years 4 months 24 days

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=96867065&loc=d3e5070-113901

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
-Paragraph 2
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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.

+ References

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-Publisher FASB
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-SubTopic 10
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-Paragraph 2
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- Definition

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).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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-Paragraph 2
-Subparagraph (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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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.

+ 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 (c)(2)(i)-(ii)
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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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- Definition

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

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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-Paragraph 2
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- Definition

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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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-Subparagraph (c)(2)(iii)(2)
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