Cash and stock-based compensation plans (Details 6) (Indexed Stock Option Plan [Member], USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Indexed Stock Option Plan [Member]
Options Outstanding	325,936	385,469	385,469
Forfeited Options	0	0	0
Expired Options	(3,542)	(4,100)
Exercised Options	(322,394)	(55,433)	0
Options Outstanding	0	325,936	385,469
Exercisable Options	0
Outstanding weighted average exercise price	$ 12.86	$ 17.98	$ 17.46
Forfeited Weighted average exercise price	$ 0	$ 0	$ 0
Expired Weighted average exercise price	$ 14.48	$ 11.87
Exercised Weighted average exercise price	$ 16.41	$ 12.12	$ 0
Outstanding weighted average exercise price	$ 0	$ 12.86	$ 17.98
Exercisable Weighted average exercise price	$ 0
Outstanding Weighted Average remaining contractual term	0 years
Exercisable Weighted Average remaining contractual term	0 years
Outstanding Aggregate intrinsic value	$ 0
Exercisable Aggregate intrinsic value	$ 1,213	$ 235

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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Number of options or other stock instruments for which the right to exercise has lapsed under the terms of the plan agreements.

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

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The number of shares reserved for issuance under stock option agreements awarded under the plan that validly exist and are outstanding as of the balance sheet date, including 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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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 of the plan that expired.

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

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