Dividends and Basic Earnings Per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Dividends and Basic Earnings Per Common Share [Abstract]
Cash dividends	$ 9,919us-gaap_DividendsCommonStockCash	$ 9,723us-gaap_DividendsCommonStockCash	$ 9,418us-gaap_DividendsCommonStockCash
Cash dividends per share of common stock (in dollars per share)	$ 12.70	$ 12.50	$ 12.10
Percentage increase in cash dividend per share (in hundredths)	1.60%
Earnings per share for the period [Abstract]
Net Income	$ 25,402us-gaap_NetIncomeLoss	$ 24,061us-gaap_NetIncomeLoss	$ 23,349us-gaap_NetIncomeLoss
Weighted Average Number of Common Shares Outstanding (in shares)	778,358	777,882	778,648
Basic Earnings Per Common Share (in dollars per share)	$ 32.64us-gaap_EarningsPerShareBasic	$ 30.93us-gaap_EarningsPerShareBasic	$ 29.99us-gaap_EarningsPerShareBasic

The increase in total per share cash dividend during the period.

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Aggregate dividends paid during the period for each share of common stock outstanding.

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Amount of paid and unpaid common stock dividends declared with the form of settlement in cash.

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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