Share-based compensation plans - Schedule of status of options, and changes during the period (Details) - Share options - Class A common shares - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
2004 stock option plans
Number of shares subject to options
Outstanding (in shares)	85,650	216,350
Exercised (in shares)	(2,548)	(29,262)
Forfeited, cancelled or expired (in shares)	(16,752)	(101,438)
Outstanding (in shares)	66,350	85,650
Exercisable (in shares)	66,350
Weighted average exercise price
Outstanding, weighted average exercise price (in dollars per share)	$ 36.30	$ 30.45
Exercised, weighted average exercise price (in dollars per share)	5.89	5.89
Forfeited, cancelled or expired, weighted average exercise price (in dollars per share)	27.75	32.59
Outstanding, weighted average exercise price (in dollars per share)	39.63	$ 36.30
Exercisable (in dollars per share)	$ 39.63
Weighted average remaining contractual life
Outstanding, weighted average remaining contractual life	1 year 2 months
Exercisable, weighted average remaining contractual life	1 year 2 months
Aggregate intrinsic value
Outstanding, aggregate intrinsic value	$ 116
Exercisable, aggregate intrinsic value	$ 116
2009 share award and incentive plan
Number of shares subject to options
Outstanding (in shares)	2,702,957	3,089,633
Exercised (in shares)	(59,833)	(100,262)
Forfeited, cancelled or expired (in shares)	(516,935)	(1,045,888)
Outstanding (in shares)	2,644,489	2,702,957
Exercisable (in shares)	1,347,734
Weighted average exercise price
Outstanding, weighted average exercise price (in dollars per share)	$ 10.97	$ 11.20
Granted, weighted average exercise price (in dollars per share)	11.61	10.48
Exercised, weighted average exercise price (in dollars per share)	9.35	8.60
Forfeited, cancelled or expired, weighted average exercise price (in dollars per share)	10.77	11.51
Outstanding, weighted average exercise price (in dollars per share)	11.17	$ 10.97
Exercisable (in dollars per share)	$ 10.91
Weighted average remaining contractual life
Outstanding, weighted average remaining contractual life	7 years 4 months
Exercisable, weighted average remaining contractual life	5 years 10 months
Aggregate intrinsic value
Outstanding, aggregate intrinsic value	$ 6,175
Exercisable, aggregate intrinsic value	$ 3,592

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
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-Subparagraph (c)(1)(iii)
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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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- Definition

For presentations that combine terminations, the number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan or that expired.

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Weighted average price of options that were either forfeited or expired.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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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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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of share options (or share units) exercised during the current period.

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