Share-based compensation Nonqualified Stock Options (Details) (Nonqualified stock options (NQSOs), USD $)	12 Months Ended
Dec. 31, 2013	Dec. 31, 2012
Nonqualified stock options (NQSOs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding at the beginning of the period (in shares)	14,000	55,500
Outstanding, at the beginning of the period (in dollars per share)	$ 20.49	$ 20.92
Granted (in shares)	0	0
Granted (in dollars per share)	$ 0.00	$ 0.00
Exercised (in shares)	(14,000)	(41,500)
Exercised (in dollars per share)	$ 20.49	$ 21.06
Forfeited (in shares)	0	0
Forfeited (in dollars per share)	$ 0.00	$ 0.00
Expired (in shares)	0	0
Expired (in dollars per share)	$ 0.00	$ 0.00
Outstanding at the end of the period (in shares)	0	14,000
Outstanding, at the end of the period (in dollars per share)	$ 0.00	$ 20.49
Exercisable (in shares)	0	14,000
Exercisable (in dollars per share)	$ 0.00	$ 20.49

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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.

+ 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 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.

+ References

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- Definition

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

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

+ References

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-Publisher FASB

-Name Accounting Standards Codification

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-Subparagraph (c)(1)(iv)(1)

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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

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- Definition

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

+ 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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- 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

Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

+ 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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