Employee benefit plans - Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended
Sep. 30, 2017	Dec. 31, 2016
Shares (in thousands)
Outstanding at beginning of period (shares)	12,379
Granted (shares)	1,773
Exercised (shares)	(1,296)
Forfeited/Cancelled (shares)	(2,910)
Outstanding at end of period (shares)	9,946
Weighted- average exercise price
Outstanding at beginning of period (in dollars per share)	$ 12.17
Granted (usd per share)	9.31
Exercised (usd per share)	1.64
Forfeited/Cancelled (usd per share)	18.19
Outstanding at end of period (in dollars per share)	$ 11.28
Aggregate intrinsic value (in thousands)	$ 33,525	$ 32,772
Exercisable (shares)	8,069
Exercisable - Weighted average exercise price (in dollars per share)	$ 11.01
Exercisable - Aggregate intrinsic value	$ 31,378

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.

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

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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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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.

+ References

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-SubTopic 10
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Weighted average price of options that were either forfeited or expired.

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(3)-(4)
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Gross number of share options (or share units) granted during the period.

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

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