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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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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).

+ 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)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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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
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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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.

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Intrinsic value of equity-based compensation awards not vested. Excludes stock and unit options.

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No definition available.

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Share-based payments (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Time-Based Restricted Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period			3 years
Performance-Based Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period			3 years
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate fair values at grant date	$ 3.9
Share-based incentives, shares issued	90
Average grant price (in usd per share)	$ 43.33
Restricted Stock Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate fair values at grant date		$ 3.0
Share-based incentives, shares issued		69
Average grant price (in usd per share)		$ 43.87