Stock Based Compensation (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2012	Mar. 31, 2011
Notes to Financial Statements [Abstract]
Percentage of eligible earnings to purchase common stock through the employee stock purchase plan	10.00%
Discount on the average closing price for the employee stock purchase plan	5.00%
Stock Based Compensation Expense Table
Stock based compensation	$ 9,954	$ 8,112
Tax effect recognized in net income	3,484	2,839
Windfall tax benefit/(shortfall), net	7,415	2,791
Employee stock options granted during the period	383,600	652,000
Weighted average assumptions used to value option grants Table
Fair value per share ($)	$ 29.65	$ 24.86
Risk-free interest rate (%)	0.82%	2.07%
Expected option life (years)	5.22	5.33
Expected volatility (%)	36.56%	35.17%
Expected dividend yield (%)	0.59%	0.59%
Cash received from exercise of options	16,873	8,607
Restricted stock awards granted during period	258,057	282,580
Weighted average fair value per share of restricted stock awards granted during the period	$ 93.62	$ 74.12
Restricted stock awards vested during period	189,678
Weighted average grant date fair value per share	$ 54.09
Weighted average vest date fair value per share	$ 94.32
Shares of stock purchased during the period by participants in the employee stock purchase plan	6,764	7,460
Amount paid for stock purchased during the period by participants in the employee stock purchase plan	$ 600	$ 579

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The weighted average fair value as of vest dates pertaining to a share-based award plan other than a stock option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares, other instruments, or cash in accordance with the terms of the arrangement.

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The quantity of shares issuable on restricted awards granted under the plan during the reporting period.

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The total consideration received during the period for purchases of shares of stock from participants of the employee stock purchase plan.

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Represents the expense recognized during the period arising from share-based compensation arrangements (for example, shares of stock, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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The total recognized tax benefit related to compensation cost for share-based payment arrangements recognized in income during the period.

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Disclosure of the aggregate tax benefit realized from the exercise of stock options and the conversion of similar instruments during the annual period.

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The cash inflow associated with the amount received from holders exercising their stock options.

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An entity may opt to disclose the discount rate from market value on offering date or purchase date that participants pay for shares. Typically, the participant's per share cost is the lower of the prices on the two dates.

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The weighted average fair value at grant for nonvested share-based awards issued during the period on other than stock option plans (for example, phantom stock plan, stock appreciation rights plan, performance target plan).

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The decrease in the number of shares potentially issuable under a share-based award plan pertaining to awards for which the grantee has gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares, other instruments, or cash in accordance with the terms of the arrangement.

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The weighted average fair value as of grant dates pertaining to a share-based award plan other than a stock option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares, other instruments, or cash in accordance with the terms of the arrangement.

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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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The period of time a share-based award is expected to be outstanding. A share-based award's expected term shall be determined based on, among other factors, the instrument's contractual term and the effects of employees' expected exercise and post-vesting employment termination behavior. An entity is required to aggregate individual awards into relatively homogeneous groups.

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The estimated measure of the percentage amount by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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An entity may opt to disclose the highest percentage of annual salary that an employee is permitted to utilize with respect to the plan.

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The quantity of shares issuable on stock options awarded under the plan during the reporting period.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Number of shares issued during the period as a result of an employee stock purchase plan.

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