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Stock compensation plans (Tables) | 12 Months Ended
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Dec. 31, 2012
Weighted-Average Assumptions used to Estimate Fair Value of Options Granted | The following weighted-average assumptions were used:
|  | 2012 |  |  | 2011 |  |  | 2010 |
Risk-free interest rate |  |  | 0.81 | % |  |  | 1.92 | % |  |  | 2.12 | %
Expected life of option |  |  | 4.85 yrs. |  |  |  | 4.70 yrs. |  |  |  | 4.66 yrs. |
Expected dividend yield |  |  | 2.28 | % |  |  | 2.34 | % |  |  | 2.22 | %
Expected volatility |  |  | 28.46 | % |  |  | 26.82 | % |  |  | 26.42 | %
Weighted-Average Assumptions used to Estimate Fair Value of Non-Vested Shares Granted | The following weighted-average assumptions were used:
|  | 2012 |  |  | 2011 |  |  | 2010 |
Risk-free interest rate |  |  | 0.37 | % |  |  | 1.04 | % |  |  | 1.25 | %
Expected dividend yield |  |  | 2.24 | % |  |  | 2.34 | % |  |  | 2.24 | %
Summary of Stock Option Activity | The following table summarizes the option activity during 2012:
|  | Shares Subject to Options |  |  | Weighted Average Exercise Price |  |  | Weighted Average Remaining Contractual Life In Years |  |  | Aggregate Intrinsic Value |
|  |  |  |  |  |  |  |  |  |  | (Dollars in thousands) |
Outstanding, beginning of the year |  |  | 1,132,546 |  |  | $ | 57.17 |  |  |  |  |  |  |  |  |
Granted |  |  | 431,667 |  |  |  | 59.85 |  |  |  |  |  |  |  |  |
Exercised |  |  | (204,082 | ) |  |  | 51.66 |  |  |  |  |  |  |  |  |
Forfeited or expired |  |  | (275,938 | ) |  |  | 60.50 |  |  |  |  |  |  |  |  |
Outstanding, end of the year |  |  | 1,084,193 |  |  | $ | 58.43 |  |  |  | 7.0 |  |  | $ | 14,037 |
Exercisable, end of the year |  |  | 558,768 |  |  | $ | 57.53 |  |  |  | 5.4 |  |  | $ | 7,772 |
Summary of Non-Vested Restricted Stock Unit Activity | The following table summarizes the non-vested restricted stock unit activity during 2012:
|  | Number of Non-Vested Shares |  |  | Weighted Average Grant- Date Fair Value |  |  | Weighted Average Remaining Contractual Life In Years |  |  | Aggregate Intrinsic Value |
|  |  |  |  |  |  |  |  |  |  | (Dollars in thousands) |
Outstanding, beginning of the year |  |  | 334,459 |  |  | $ | 56.60 |  |  |  |  |  |  |  |  |
Granted |  |  | 178,690 |  |  |  | 56.95 |  |  |  |  |  |  |  |  |
Vested |  |  | (78,753 | ) |  |  | 45.54 |  |  |  |  |  |  |  |  |
Forfeited |  |  | (69,608 | ) |  |  | 54.92 |  |  |  |  |  |  |  |  |
Outstanding, end of the year |  |  | 364,788 |  |  | $ | 55.77 |  |  |  | 1.3 |  |  | $ | 26,013 |

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- Definition Tabular disclosure of the changes in outstanding nonvested shares. + 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)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of the number and weighted-average exercise prices (or conversion ratios) for share options (or share units) that were outstanding at the beginning and end of the year, vested and expected to vest, exercisable or convertible at the end of the year, and the number of share options or share units that were granted, exercised or converted, forfeited, and expired during the year. + 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) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of the weighted-average grant-date fair value of equity options or other equity instruments granted during the year. + 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 (d)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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