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Significant accounting policies - Stock-based Compensation (Details) (Restricted Share Unit (RSU))	12 Months Ended
	Dec. 31, 2014
Restricted Share Unit (RSU)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award service period	2 years
Award vesting period	3 years
Number of trading days used to determine weighted average trading price of common shares	5 days
Compensation expense recognition period	3 years

X

- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Number of Trading Days Used to Calculate Weighted Average Trading Price of Common Stock

+ References

No definition available.

+ Details

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X

- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Period for Recognition

+ References

No definition available.

+ Details

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X

- Definition

Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ 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 (a)(1)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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X

- Definition

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.

+ 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 (a)(1)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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