Cash and stock-based compensation plans (Details 7) (Indexed Stock Option Plan [Member], USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011
Indexed Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options Outstanding	325,936	385,469
Forfeited Options	0	0
Expired Options	(3,542)	(4,100)
Exercised Options	(322,394)	(55,433)
Options Outstanding	0	325,936
Outstanding weighted average exercise price	$ 12.86	$ 17.98
Forfeited Weighted average exercise price	$ 0	$ 0
Expired Weighted average exercise price	$ 14.48	$ 11.87
Exercised Weighted average exercise price	$ 16.41	$ 12.12
Outstanding weighted average exercise price	$ 0	$ 12.86
Outstanding Weighted Average remaining contractual term	0 years
Outstanding Aggregate intrinsic value	$ 0

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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.

+ 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)(iv)(4)

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

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- Definition

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.

+ 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)(iv)(3)

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

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- Definition

Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

+ References

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- Definition

Number of options outstanding, including both vested and non-vested options.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

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- Definition

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

+ References

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-Publisher FASB

-Name Accounting Standards Codification

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- Definition

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.

+ References

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

-Publisher FASB

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-Subparagraph (e)(1)

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- Definition

Weighted average price at which option holders acquired shares when converting their stock options into shares.

+ References

No definition available.

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- Definition

Weighted average price at which grantees could have acquired the underlying shares with respect to stock options of the plan that expired.

+ References

No definition available.

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

+ References

No definition available.

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- Definition

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

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3

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