Cash and stock-based compensation plans (Details 4) (Stock Purchase Options [Member], USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Stock Purchase Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options Outstanding	187,745	423,672	915,566
Granted Options	315,971	0	182,420
Forfeited Options	(671)	(9,780)	(231,639)
Exercised Options	(111,349)	(226,147)	(442,675)
Options Outstanding	391,696	187,745	423,672
Exercisable Options	33,803
Expected to vest Options	357,893
Outstanding weighted average exercise price	$ 14.90	$ 13.83	$ 12.87
Granted Weighted average exercise price	$ 25.15	$ 0	$ 18.93
Forfeited Weighted average exercise price	$ 18.57	$ 18.18	$ 15.82
Exercised Weighted average exercise price	$ 13.18	$ 12.76	$ 12.90
Outstanding weighted average exercise price	$ 23.65	$ 14.90	$ 13.83
Exercisable Weighted average exercise price	$ 15.53
Expected to vest Weighted average exercise price	$ 24.42
Outstanding Weighted average remaining contractual term	5 years 7 months 13 days
Exercisable Weighted average remaining contractual term	2 years 1 month 10 days
Expected to vest Weighted average remaining contractual term	5 years 10 months 17 days
Outstanding Aggregate intrinsic value	$ 2,526
Exercisable Aggregate intrinsic value	493
Expected to vest Aggregate intrinsic value	$ 2,033

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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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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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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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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.

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

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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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Amount by which the current fair value of the underlying stock exceeds the exercise price of fully vested and expected to vest options outstanding.

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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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As of the balance sheet date, the weighted-average exercise price for outstanding stock options that are fully vested or expected to vest.

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Weighted average remaining contractual term for fully vested and expected to vest options outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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 price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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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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