Stock option plan (Details 2) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Stock options outstanding and exercisable
Outstanding, Number of options (in shares)	3,432,972
Exercisable, Number of options (in shares)	764,277
Additional compensation recognized due to modifications to the options granted	$ 1.9	$ 0.6
$10.59-$10.85
Stock options outstanding and exercisable
Range of Exercise Price, Lower (in dollars per share)	$ 10.59
Range of Exercise Price, Higher (in dollars per share)	$ 10.85
Outstanding, Number of options (in shares)	1,752,630
Outstanding, Weighted-Average Exercise Price (in dollars per share)	$ 10.85
Outstanding, Weighted-Average Remaining Contractual Life	8 years 4 months 13 days
Exercisable, Number of options (in shares)	704,958
Exercisable, Weighted-Average Exercise Price (in dollars per share)	$ 10.85
$10.85-$20.76
Stock options outstanding and exercisable
Range of Exercise Price, Lower (in dollars per share)	$ 10.85
Range of Exercise Price, Higher (in dollars per share)	$ 20.76
Outstanding, Number of options (in shares)	114,425
Outstanding, Weighted-Average Exercise Price (in dollars per share)	$ 19.42
Outstanding, Weighted-Average Remaining Contractual Life	8 years 10 months 24 days
Exercisable, Number of options (in shares)	28,675
Exercisable, Weighted-Average Exercise Price (in dollars per share)	$ 19.72
$20.76 - $490.80
Stock options outstanding and exercisable
Range of Exercise Price, Lower (in dollars per share)	$ 20.76
Range of Exercise Price, Higher (in dollars per share)	$ 27.05
Outstanding, Number of options (in shares)	982,796
Outstanding, Weighted-Average Exercise Price (in dollars per share)	$ 26.91
Outstanding, Weighted-Average Remaining Contractual Life	9 years 1 month 13 days
Exercisable, Number of options (in shares)	350
Exercisable, Weighted-Average Exercise Price (in dollars per share)	$ 27.05
$490.80-735.60
Stock options outstanding and exercisable
Range of Exercise Price, Lower (in dollars per share)	$ 27.05
Range of Exercise Price, Higher (in dollars per share)	$ 50.45
Outstanding, Number of options (in shares)	554,400
Outstanding, Weighted-Average Exercise Price (in dollars per share)	$ 39.25
Outstanding, Weighted-Average Remaining Contractual Life	9 years 7 months 13 days
Exercisable, Number of options (in shares)	2,968
Exercisable, Weighted-Average Exercise Price (in dollars per share)	$ 40.45
$1,149.60
Stock options outstanding and exercisable
Range of Exercise Price, Lower (in dollars per share)	$ 50.45
Range of Exercise Price, Higher (in dollars per share)	$ 1,149.60
Outstanding, Number of options (in shares)	28,721
Outstanding, Weighted-Average Exercise Price (in dollars per share)	$ 569.97
Outstanding, Weighted-Average Remaining Contractual Life	4 years 3 months 26 days
Exercisable, Number of options (in shares)	27,326
Exercisable, Weighted-Average Exercise Price (in dollars per share)	$ 585.05

An excess of the fair value of the modified award over the fair value of the award immediately before the modification.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (h)(2)(iii)

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

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- Definition

Weighted average exercise price as of the balance sheet date for those equity-based payment arrangements exercisable and outstanding.

+ 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)(1)(iii)

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

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- Definition

The floor of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range.

+ 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 (g)

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

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The number of shares reserved for issuance pertaining to the outstanding exercisable stock options as of the balance sheet date in the customized range of exercise prices for which the market and performance vesting condition has been satisfied.

+ 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)(iii)

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

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- Definition

The number of shares reserved for issuance pertaining to the outstanding stock options as of the balance sheet date for all option plans in the customized range of exercise prices.

+ 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)(1)(i)-(ii)

-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 could acquire the underlying shares with respect to all outstanding stock options which are in the customized range of exercise prices.

+ 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)(1)(i)-(ii)

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

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- Definition

Weighted average remaining contractual term of outstanding stock options, 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

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-SubTopic 10

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The ceiling of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range.

+ 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 (g)

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