Stock-based Compensation (Summary of Stock Option Activity) (Details) - $ / shares	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Outstanding, beginning of period, Shares	1,062,177	1,396,152
Granted, Shares	294,603	460,610
Exercised, Shares	(149,043)	(180,940)
Forfeited or cancelled, Shares	(50,491)	(16,567)
Outstanding, end of period, Shares	1,157,246	1,659,255
Outstanding, beginning of period, Weighted Average Exercise Price	$ 75.53	$ 68.63
Granted, Weighted Average Exercise Price	110.68	90.85
Exercised, Weighted Average Exercise Price	71.74	59.50
Forfeited or cancelled, Weighted Average Exercise Price	88.48	80.40
Outstanding, end of period, Weighted Average Exercise Price	$ 84.40	$ 75.68

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

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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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