Stock-Based Compensation - Option Rollforward (Details) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll [Forward]
Beginning Balance	75,251	107,455
Options Granted	37,048	29,302
Options Exercised	(25,224)	(57,137)
Options Forfeited	0	(3,601)
Options Expired	0	(768)
Ending Balance	87,075	75,251
Options Exerciseable	18,696	11,840
Share Based Compensation Arrangement By Share Based Payment Award Options Forfeitures And Expirations In Period Weighted Average Exercise Price [Abstract]
Outstanding at beginning of year	$ 44.49	$ 31.23
Options Granted	$ 73.47	$ 58.26
Options Exercised	$ 36.65	$ 27.12
Options Forfeited	$ 0.00	$ 37.81
Options Expired	$ 0.00	$ 37.37
Outstanding at End of Period	$ 59.09	$ 44.49
Options Exerciseable	$ 44.38	$ 28.42
Share Based Compensation Arrangement By Share Based Payment Award Options Additional Disclosures [Abstract]
Weighted Average Remaining Contractual Term, Outstanding	5 years 2 months	5 years 2 months
Weighted Average Remaining Contractual Term, Exercisable	4 years 1 month	3 years 11 months

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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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Number of options or other stock instruments for which the right to exercise has lapsed under the terms of the plan agreements.

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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.

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

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No definition available.

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

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No definition available.

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

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

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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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