Equity - Performance shares (Details) $ / shares in Units, shares in Thousands, $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2019 USD ($) item $ / shares shares	Dec. 31, 2018
Performance Shares
Share-based compensation
Number of tranches \| item	2
Vesting terms		3 years
Granted (in shares)	70
Weighted-average fair value per share, Granted (in dollars per share) \| $ / shares	$ 92.57
Unrecognized compensation cost \| $	$ 6.0
Remaining requisite service period (in years)	2 years 3 months 18 days
Cancelled (in shares)	3
Market-based performance shares
Share-based compensation
Percentage of share-based compensation award	50.00%
Performance shares calculation period (in years)	3 years
Vesting terms	3 years
Market-based performance shares \| Minimum
Share-based compensation
Performance shares available for vesting (as a percent)	0.00%
Market-based performance shares \| Maximum
Share-based compensation
Performance shares available for vesting (as a percent)	200.00%
Internal-based performance based
Share-based compensation
Percentage of share-based compensation award	50.00%
Performance shares calculation period (in years)	3 years
Vesting terms	3 years
Internal-based performance based \| Minimum
Share-based compensation
Performance shares available for vesting (as a percent)	0.00%
Internal-based performance based \| Maximum
Share-based compensation
Performance shares available for vesting (as a percent)	200.00%
Performance Shares Award Granted in 2016
Share-based compensation
Award pay out achieved (as a percent)	0.00%

Represents the vesting percentage achieved.

No definition available.

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- Definition

The length of time used to calculation the final payments for performance shares.

+ References

No definition available.

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- Definition

Number of tranches in share-based compensation award type.

+ References

No definition available.

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- Definition

Percentage of share-based compensation award.

+ References

No definition available.

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- Definition

Minimum period the individual is required to perform services to be fully vested under the deferred compensation arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 710
-SubTopic 10
-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=6409961&loc=d3e20487-108367

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

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- Definition

Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

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- Definition

Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

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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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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(3)
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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

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-Name Accounting Standards Codification
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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

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Percentage of vesting of award under share-based payment arrangement.

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