Stock Option Plans - Summary of option activity (Details) (Stock Options, USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Stock Options
Shares
Outstanding, beginning of year	101,336	107,338
Granted
Exercised	(15,101)	(6,002)
Forfeited or expired	(400)
Outstanding, end of year	85,835	101,336
Exercisable, end of year	23,600	18,151
Weighted Average Exercise price
Outstanding, beginning of year	$ 15.63	$ 15.60
Granted
Exercised	$ 15.53	$ 15.04
Forfeited or expired	$ 15.65
Outstanding, end of year	$ 15.65	$ 15.63
Exercisable, end of year	$ 15.65	$ 15.55
Weighted Average Remaining Contractual Term
Outstanding, end of year	7 years 3 months	8 years 2 months 1 day
Exercisable, end of year	7 years 3 months	7 years 9 months 11 days
Aggregate Intrinsic Value
Outstanding, end of year	$ 630,887	$ 397,049
Exercisable, end of year	$ 173,460	$ 72,628

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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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For presentations that combine terminations, 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 or that expired.

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Weighted average price of options that were either forfeited or expired.

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

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

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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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