Stock-based compensation - Stock Option Activity (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Shares
Outstanding at beginning of period (shares)	25,134
Granted (shares)	420
Exercised (shares)	(4,143)
Forfeited/Cancelled (shares)	(74)
Outstanding at end of period (shares)	21,337
Weighted- average exercise price
Outstanding at beginning of period (in dollars per share)	$ 6.62
Granted (in dollars per share)	$ 45.56
Exercised (in dollars per share)	$ 1.44
Forfeited/Cancelled (in dollars per share)	$ 20.91
Outstanding at end of period (in dollars per share)	$ 8.34
Granted, Weighted average fair value at grant date (in dollars per share)	$ 22.47
Exercised, Total intrinsic value of exercises	$ 203,784
Aggregate intrinsic value	756,135	1,425,339
Exercisable (shares)	14,678
Exercisable - Weighted average exercise price (in dollars per share)	$ 2.68
Exercisable - Aggregate intrinsic value	597,831
Vested and expected to vest (shares)	20,966
Vested and expected to vest - Weighted average exercise price (in dollars per share)	$ 8.08
Vested and expected to vest - Aggregate intrinsic value	$ 748,068

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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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.

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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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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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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.

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

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Gross number of share options (or share units) granted during the period.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of fully vested and expected to vest options that are exercisable.

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As of the balance sheet date, the weighted-average exercise price (at which grantees can acquire the shares reserved for issuance) for exercisable stock options that are fully vested or expected to vest.

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As of the balance sheet date, the number of shares into which fully vested and expected to vest stock options outstanding can be converted under the 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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Number of share options (or share units) exercised during the current period.

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